Hello, I'm Apophis-Set and I'll be writing how to solo each boss:
(Keep in mind this is mainly for soloing, I'll be writing a thread on how to do with a group later :3)
(Also keep in mind that for snarbolax, I'm assuming you have 2 star equipment, 2 weapon slots and no trinket slots. For jelly king and roarmulus twins, I'm assuming you have 3-4 star equipment, 3 weapon slots and no trinket slots. For vanaduke, I'm assuming you have 5star equipment, 4 weapon slots and 2 trinket slots. If you have a different star set, just assume the weapon in the same line)
(Also assuming you're doing the mission)

Snarby=snarbolax
JK=jelly king
RT=roarmulus twins
Vana=vanaduke

Snarbolax:

This is by far the easiest boss in the game.
The boss floors consist of mainly beasts, with a couple constructs and undeads thrown in.
As beasts are relatively difficult to hit with guns, I suggest bringing only swords to this boss.
A good loadout would be:
Flourish/Snarble barb (for high piercing dps) (snarble barb is only obtainable after getting 10 snarbolax tokens)
This sword has a boost forward by about 1-2 squares on the 3rd hit, so be careful while using it, lest you dive into spikes. You can shield cancel, which is where you swing and then quickly hit shield right after you hit attack. This cancels out the animation time, and with some practice, can make your attacks a lot faster.
Brandish/calibur (for taking down those constructs that are resistant against piercing) (brandish upgrades to either normal/elemental with a status, or shadow, while calibur stays normal damage)
Wolver cap (because you are using swords, so the sword damage bonus comes in handy. Anything with normal protection will do)
Wolver coat (same reason as wolver cap)
Bristling buckler/defender/owlite shield (sword damage bonus from bristling buckler... Any shield with normal protection will do, really) (bristling buckler is only obtainable after getting 10 snarbolax tokens)
You don't really need anything except for 3 health capsules here...
Note: you can break tufts of grass for a chance at a couple crowns or a heart, but bushes don't drop anything, so just ignore them

For the first floor, get your flourish out, 3 wolvers are going to spawn! Take care of them, and then head onward! You will come to this place where the floor forms an N. there is a gate to the right, and a passage all the way upwards. As you head into the N, 1 wolver, 1 chromalisk (lizard/frog things), and 1 silkwing (flying mothlike healer) will spawn. Kill them, taking care not to step into the spikes, and then head up the upper passageway, bearing right, then down. There will be a switch there. Hitting the switch will mean opening that gate you saw previously, but be careful, because once you hit it, 2 wolvers spawn north of the switch, on the passageway you just came from. Kill them, then head thru the gate that just opened.

As you head into the gate, the passage will head downward and send you into a room. This room has no spikes, but you can see a snarbolax in the middle! As you approach it, the snarbolax will roar and then spawn 3 wolvers, and disappear. This room has two crystals in the upper right corner, as well as a button, which opens the gate to the south. There is a bell at the north side of the room, which when hit by a weapon, stuns nearby monsters. Kill the 3 wolvers, break the crystals, press the switch, and depart downward. You will enter a room with a party button in the middle, and a row of spikes in each direction. There is a quick appearance by snarbolax, but ignore it (they may have removed this). Charge up your weapon (if flourish, don't charge, as that charge attack is difficult to land on zombies, the next spawn, without running into spikes, unless you are confident of your skill). Once you step on the button, 3 zombies will spawn, 1 on the left and 2 on the right. These are simple to kill. As soon as you kill those, 5 kats will spawn, 3 on top and 1 on both left and right. What they do is they will charge at you, and THEN use an attack. If you are near to it, it will try to bite you. The trick is, dont shield away from them until AFTER they charge, because their charge doesn't do damage. As soon as you kill those, gun puppies will spawn at the top, left, and right of the room, 1 on each side. (IMPORTANT: do not use flourish here, use your other weapon, as gun puppies are constructs, which are resistant to piercing) Their attack is spitting one bullet. You can either dodge the bullet, if you are confident of your movement skills, or you can shield and then attack once or twice, and then shield again. Once you killed them, the gate leading to the upper right will open, and you can break open the heart boxes to get hearts. Head up, and the next room you find yourself in is rather large. There are rows of spikes, and where there are gaps, there is either 3 3-hit-to-break blocks or 3 explosive cubes and 1 monster cage. You have to stand at a safe distance, just barely, and use your flourish/snarble barb's first swing, and then shield as quickly as possible, just in case. The monster that spawns out of the monster spawned can be either a wolver, a chromalisk, a gun puppy, or a rocket puppy, which is like a gun puppy that shoots rockets. My advice is to break all the monster spawners, by either exploding nearby explosive blocks or breaking them with your weapon.

As you head up, the room will turn into a place that looks somewhat like C.N with the . being a wall. There will be a bell there, as well as an alpha wolver and two wolvers. A line of explosive blocks block your way to continue into the middle of the N, and a gun puppy is sitting behind the explosive block line. Keep going, and you'll find yourself in a room with 3 crystals and a passage leading to the right. As you head right, there is another room with a party button. IMPORTANT: when you get in the party button, make sure you're shielding, because snarbolax will pop out of nowhere and run over the party button, stunning you if you're not shielding. While fighting, watch the spikes. First, 4 wolvers will spawn. After killing them, 2 wolvers, an alpha wolver, and a gun puppy at the top of the room will spawn. After killing them, two gun puppies at top left and top right will spawn, as well as a lumber (giant tree ogre who attacks with a branch arm) will spawn in the upper right corner. The trick to killing lumbers is to wait until it's about to attack you (you can tell from the red marks that will show) and then dashing to the side and attacking it over and over until its about to attack you again. Repeat. After killing them, you head upward and you reach the end area of the floor, where there are six prize boxes and snarbolax is waiting in front of the elevator. As you approach, he dives underground. First floor is complete.

At the beginning of the second floor, head up and right to see the passageway split into two, one heading to upper right and one heading to lower right. Three chromalisks will be waiting, and once you finish them off, head to the upper right passageway. You will see a gate that requires a key at the upper right, and a set of unbreakable blocks with a vanishing block. Break the vanishing block to get rid of the unbreakable blocks. You will see a button, once you press it, a gate at the end of the previous lower right passageway will open, allowing you access to the key to open the gate that needs a lock. As soon as you press the button, two wolvers will spawn to the north of you. Take them out, then head back left and down the lower right passageway, shaped like UN, and a wolver is waiting at the second curve, at the top, a gun puppy is waiting at the third curve, at the bottom, a wolver is waiting at the fourth curve, at the bottom, and a gun puppy is waiting at the fifth curve, near the top. Kill them all, then head right to get the key and bring it back to the upper right passageway to open the gate. As you head up, you'll see snarbolax dash from right to left across the room, which has two bells on the left and the right. A double gate set, both needing keys, are at the top. 6 wolvers will spawn, and once you kill them all, the gates will open, 2 leading left and 2 leading right. (I'm going to write the left side first).

Head to the bottom left, press the button, and three gun puppies will spawn in the Z shaped passageway. Once you take them out, head upward. (IMPORTANT: as you head upward, shield, because snarbolax will come charging out and stun you if you aren't shielding). Once you get near enough to the top, 3 zombies will spawn. Kill them, the break the blocks until you find the one with the button under it to open the two gates at the top left and top right of the room. Once the gates are open, the spikes will begin circulating, the spikes on the right are timed so that you can follow the empty space between two spikes up to go above the room, and you will see the key. Pick up the key, and this time head left and down, and follow the space between two spikes. Go put the key in the first gate in the 6-wolver-spawn room, and then head right. The right side is a bit trickier, because there are a series of three spikes followed by respawning exploding blocks, so you have to time it right and stand in the right position with the flourish/snarble barb to get across. Once across, hit the disappearing block, walk through, and head upward towards the upper left disappearing block. First, press the switch at the right side, then head up to break the disappearing block. Once hit, all the walls will disappear, and you go on a wild rampage killing all the monsters there, consisting of: 3 chromalisks, 4 monster spawning blocks, a wolver, and after you hit the disappearing block, a gun puppy near the bottom-right/center area along with 2 wolvers. Once done, head towards the bottom left with the key, break the heart boxes, and go to put the key into the 2nd gate. 2 zombies will be near the gate. Kill them, and head upwards.

You will emerge in a room with, seemingly, lots of exploding boxes. In the upper right corner, a switch sits, and a pot dispenser. A row of spikes lies across the right side to the center of the room, and north of that, a row of exploding boxes. The trick here is to throw a pot at the switch, which turns off the spikes for about 5 seconds. You have to use your flourish/snarble barb to break the exploding cubes, and head upwards and a bit left, then downwards, hit the respawning disappearing block, press the button, which loweres the gate, and carry the key out to put somewhere else. As soon as you press the button, 3 wolvers and an alpha wolver spawn, and once you killed them, repeat the situation with the pot and the switch, except this time you carry the key through and use it on the gate just above the room. Head upward and enter the final room before the boss.

You will see snarbolax at the north side of the room, and when you're close enough, he burrows underground. 2 bells lie at the left and right of the room. As you step onto the party button, 2 chromalisks at the south and west spawn, a wolver at the east spawns, and an alpha wolver at the north spawns. You can lure them near the bells, then hit the bells to stun them. After they are all dead, two alpha wolvers and two wolvers spawn at the north side of the room, the alpha wolvers being on the side of the conga line they spawn in. Once you killed them off, 3 gun puppies at the bottom, upper left, and upper right spawn, as well as an alpha wolver. The trick is to try and agro only 1 gun puppy at a time, or to just take on the alpha wolver first and then worry about the gun puppies. Once you're done, go up, break the 6 boxes, and go down to the boss floor.

On the boss floor, make sure you heal up at the health pads. The boss room is basically a square room with a bell in the middle, and a rectangle of spikes just north of the bell. As soon as you step on the party button, snarbolax will spawn. IMPORTANT: while he is black, he is invincible. He will attack from a distance until he shows the red markings similar to when wolvers attack, and then start lunge-biting at you. Before he does, it is advisable that you stand right in front of the bell with your shield up, for the reason of which when snarbolax does his lunge-biting attack, he heads for you, and if you're near the bell, he will go near the bell. Once he is stunned, he turns yellow, and that's when he's vulnerable; attack him with all your might. Don't forget to keep an eye on your health bar. Once you killed him, he shrivels up and emits purple explosions as he roars, and then the gate at the top opens and a token falls out of the sky. Note: walk through the token until it begins to spin, because if it doesn't spin, it means you never picked it up. Go up, claim your prizes, and if it's your first time, use action on the blue thing with the exclamation mark to collect an artifact. Congratulations, you just soloed snarbolax!

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Jelly King:
This is the second easiest boss in the game, only slightly more difficult than snarbolax.
The boss floors consist of mainly jellies, with a couple constructs and a couple healing silkwings (fiends) scattered through the maps.
A good loadout would be:
Nightblade/Silent Nightblade (does shadow/normal damage, very useful against slimes)
Kilowatt Pulsar/Gigawatt Pulsar (for taking down those constructs from a distance, and for crowd control. While soloing, it is important to have a crowd control weapon, capable of keeping enemies away. The pulsar line can cause a lot of knockbacks to the targeted monsters, and can get you out of a pinch)
Fireburst Brandish/Blazebrand (for the status infliction of fire, shock version is acceptable, although fire charge attack here is better, because enemies hit by charge attack will ride back on the row of explosions, but shock interrupts monsters, limiting the maximum damage output of the charge attack)
Dusker Cap (for sword damage and for piercing/normal protection. Gunslinger hat is acceptable, as when you solo you tend to use guns more often, unless you're very confident of your skill with swords)
Dusker Coat (same reason as dusker cap)
Twisted Targe/Great Defender/Swiftstrike Buckler (if you don't care too much about the difference of 10 damage per hit, go with great defender for slightly more protection than twisted targe. Swiftstrike buckler has the least protection, but it gives ASI high to all your weapons, and so if you are better at dodging than shielding, you should consider swiftstrike buckler.
During the floors before the boss, stockpile on certain potions, but don't use them until you get to the boss floor.
The ideal things are:
3 health pills
3 poison vials
3 of each of (2 out of 4 of fire/shock/stun/curse, fire and curse preferred)

More to come... :3