Hardest when alone: Giant lichen colony
Hardest when in groups: Devilites/Greavers

Easiest when alone: Silkwing/Trojan/Gremlin Knocker/Wolver/Gremlin Menders
Easiest when in groups: Wolvers/Gremlin Knocker

Silkwings need some speed/hp buff. they are way too easy and dont pose any threat. Gremlin Knockers need to attack faster. Wolvers need serious buffs to attack speed, dmg and hp. Alpha wolvers need their homing bite back.

Hardest: Torn between Construct and Fiend. Construct has Mecha Knights, Ice Lumbers and Rocket Puppies, Fiend has Greavers.
Easiest: Beast, clearly.

1. Fiends.
2. Tie between Gremlins, Constructs, and Undead.
3. Tie between Slimes and Beasts.

Monster that needs buffed: Beasts. Give Wolver and Alpha bites tracking abilities.

1. Fiend;
2. Construct;
3. Undead;
4. Gremlin;
5. Jelly;
6. Beast;

Fiends only reach first because even with good guns they pose a decent challenge -they're not by any means "hard", you guys just doing it wrong.

Also wolvers with homing bites and faster chromalisks might make the beast class more interesting in combat.

@Gianor Nope, I knew that. They're still tougher than most other enemy classes.

1.fiend: best range attackers, best close-range, and one that can be hard to deal with.(worst with lag)
2.gremlin: bombers are annoing, they have the best healers(one of the main things that bring up the class), and good close-range.
3.construct: mechs can be anniong and rocket turrets are hard.
4.undead: good turrets, kats are good.
5.slime: best turrets, good specials, bigger litchen is good, but overall lackluster.
6. beast: slowest attackers, least diversity, boring.

Split it all up by playstyle exploitability and frequently-accompanying families:

• Slime: For regular jellies and lichens, keep moving and you will not get hurt (gunners and bombers win every time). Both two-swing weapons used with shield cancel and well-placed Cutter-series swords can counter their attack timing without much trouble. Polyps don't indicate at whom they are aiming, so there's extra challenge there; they also wear down shields quickly en masse. Huge Lichen Colonies are a nightmare if you don't have a strong, fairly quick knockback source.

  They are most often found with Beasts and Constructs; a two-swing sword will keep both slimes and Mecha Knights at bay while any gun will distract ruthless Wolver attacks. Any slime level makes Piercing damage protection a natural choice without need to worry about other things--though an Owlite or Drake Scale Shield might be helpful for Elemental complications.

• Beast: Wolvers have been nerfed so they're a bit less dextrous and you can even use an Autogun-series handgun on a Tier 3 Wolver. Frequent shooting will drastically reduce how much they attack. However, a pack including an Alpha Wolver will hit hard if you do not watch well for attacks. People often forget Chromalisks, but they are pretty resistant to attack disruption and the spit attacks from elemental ones can complicate an enemy-filled arena. Both are easy prey for a damage bomber, even solo. In any case, a proficiently-used Piercing sword works on either: quick attacks for wearing down Chromalisk packs and full combos to prance around Wolvers and quickly end individual Chromalisks.

  They accompany Slimes and occasionally Gremlins and Constructs. If you luck out and get just beasts in your level, your weapon and armor choice is very simple. Otherwise, you may want an Elemental-protecting shield for stray projectiles. Ensure you have full Normal protection if you have difficulty with Gremlin Thwackers.

• Construct: These form a diversity of enemies, but aside from the lowly Scuttlebots they share one trait: being terrifying in large numbers. There are a good number of ways to handle the complex Mecha Knights: Giant sword attack? Vortex? Gun and backstab? However, inexpert swordsmanship will get you slaughtered. The best techniques for handling them do not mix well with the loathsome Lumbers, which are best repelled or range-attacked when found in a large fray. Even a single Lumber can spell trouble for beginners, and they do not play well with the teammates of Pulsar-series abusers! Retrodes are barely more threatening than Scuttlebots as long as you keep an eye out for their eye-beam attacks. The most troublesome aspect of Gun Puppies is their tendency to be found in large numbers; otherwise, they are the easiest of the turret enemies. Tier 3 Rocket Puppies, however, are so terrifying that each and every occurrence will have the whole party on high alert.

  Since Constructs can show up pretty much anywhere but a pre-determined handful of levels, people tend to bring Elemental weapons to deal with any they find. You are pretty much guaranteed to find some in a Clockwork Tunnels level, and you may find Elemental-resisting Gremlins as well. If you have Gremlin troubles, a Shadow weapon may deal with both in a pinch. Proficient use of the shield makes a big difference in your opinion on Gun Puppies, whether accompanied by other enemies or not. Gun Puppies occasionally occupy setups where they must be range-attacked; if your only gun is piercing, you will be quite annoyed in these situations.

• Undead: Zombies are generally too slow to pose much of a threat (and even when they were faster, they weren't too threatening to a good shielder); their pesky breath attack can be disrupted very easily as well. Howlitzers are merely Shadow-damage Gun Puppies with an explosive surprise (usually more threatening to an unassuming teammate than the original attacker). Spookats are fast enough to cause serious problems on high-latency networks, and not everybody is good at timing and tracking the movement of packs. Even for the experienced Spookat hunter, Spookats not alerted all at once and thus not in phase with one another can pose a massive threat. All of these enemies tend to be wide open for status attacks (except Curse) and none resist ranged attacks.

  Since they are almost always in cahoots exclusively with other Shadow monsters, that may be all the protection you need (though Elemental is also not a bad idea). Bringing an Elemental sword to such a level is never a bad idea. It is exceedingly rare to want to use a Shadow weapon in a level where undead are found. I think they would be a bit more challenging if found with other Elemental-resisting monsters more frequently (like in the Gloaming Wildwoods). However, the Candlestick Keep steps up the challenge of Undead several notches by forcing you to think and attack on your feet. Grimalkins can be faked out or repelled but they otherwise interfere with undead attack patterns (even Zombie breath) to create a perfect storm.

• Gremlin: Gremlins, like Constructs also make up a diverse family of attackers. They almost necessitate using a variety of attacks to handle effectively. Gunners have a very hard time with Thwackers (and used to be all but impossible to beat that way in Tier 3). The Tier 3 Thwacker 360-degree spin can end just about any knight's plans if used often. Demos are rather difficult to handle any other way. The new versions in Operation: Crimson Hammer show how annoying they can be in combination. The Flamethrowers are pathetically weak and simple, however, and the wand-wielders ('Knockers?' seriously?) are rarely described as anything but cute. And then there are the Menders... Gremlins are the only enemies that make an effort to avoid bomb blasts, so there is no one best way to handle them all. A saving grace is that they will go into a cowardly derp-mode when low on health, which is easy to spot and not threatening.

  While they can appear with Slimes, Beasts, or Constructs, the Gremlin/Construct pairing is by far the most common; pesky Mecha Knights can be found in every Deconstruction Zone. You can probably get away with using only Shadow damage weapons and special-use weapons in a level, with an Elemental or Normal gun for backup if desired. Menders can show up in a lot of tough situations and will screw up your attacking priorities. Since they often stay in the center of an attacking mass, they demand a hard-hitting or scattering approach when not isolated right away. Menders used to revive more often and even attack, back when they were probably the most annoying enemies in the whole game. Any one Gremlin can change the optimal battle plan for a mixed group of monsters, and often experience to recognize attacks and opportunities is the best solution here.

• Fiend: The one enemy family that clearly does not smoothly scale in difficulty from Tier to Tier! Though most enemy classes get faster in harder tiers, the fiends are already fairly fast to begin with (however weak and klutzy) and only get faster (while losing much of the klutziness). Their only saving grace is being pretty weak to attack interruptions--but good luck setting up interrupting attacks safely! Greavers should really be called Griefers because of how they will make mincemeat of anyone even slightly unprepared for them. In harder tiers they will leave status dust to force you to either keep shielding or move back; in Tier 3 their bonus projectile attacks will annoy everyone rather than just the bite victim. Greavers are actually a little easier to handle solo, especially if you can get them all to strike at once so it counts as one shield hit. Devilites are nobody's friend, and their annoying quality is already obvious in Tier 2 but becomes overwhelming in Tier 3 with the ability to constantly attack. These enemies are the hardest to solo because their attacks have no regular phase (meaning they will wear down shields quickly)...but they are helpless to a strong piercing attack from any direction. A coordinating person can potentially shield for a Snarble Barb-series bomb used by a teammate and effortlessly unlock free hits on nearby enemies. Silkwings just heal and aren't all that special. Trojans are pretty much in a class of their own, but getting damaged severely by a Trojan is unlikely as long as you keep moving and stay out of corners.

  Fiends may be found with undead but rarely with anything else, except for Silkwings which may accompany Beasts and some Slimes. The fearsome devilites frequently get levels all to themselves, and their arenas are already some of the nastiest in the game (seriously, why can't other enemies get setups this challenging?). Devilites are considerably less threatening when you can separate them from their Pit Bosses; conversely, moving the Pit Bosses with shield bounces will help to lure other Devilites out from groups. In non-Devilite-specific levels, you won't often find the Pit Bosses--in which case occasional Devilites can be downgraded to regular nuisances. All Fiends become more manageable with a carefully-applied Piercing weapon and a Shadow-defense shield. Whatever weapon you use, make sure it can KO the enemy in as few attacks as possible because you will not want these fights to be prolonged.

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If you ask me, Fiends rightly deserve their 'hardest enemy' spot--but their level layouts have a fair bit to do with their difficulty. Candlestick Keep has gone a fair way in making Undead scary again, and Spookats are fast enough to mess with laggy teammates; so I would rate them second. Gremlins and their diverse attack styles make the third spot (they probably had the #2 spot when the game first came out). Then you have pesky constructs, persistent slimes and finally overly-predictable beasts.

Fiends are hardest..... But also easiest
order easiest to hardest

My order is
1.fiend(Trojan/silkwing)
2.undead
3.beast
4.construct/gremlin
5.jellys
6.fiend(devilite/greiver)

In terms of design, fiends are obviously the hardest- they have the most advanced AI, and have very powerful attacks. Grievers force you to deal with them, leaving room for things like lumbers to smack you as they follow you and status you. Devilites are devilites- they run around and throw things and dodge your blows.... trolljams I really don't think are that bad, although their AI is made to basically wreck AFK players, and they can easily mess up newer players who aren't familar with them.

The easiest monster family is by far the beast family- it consists of 2 easily defeated monsters, wolvers which have but one easily dodged attack, and chromas, which are slow moving and uncomplicated to fight. Dark briar barrage allows you to almost stand still to kill them- I usually just walk through wolver dens with DBB out breaking the bushes. I barely even notice the beasts... the only way beasts are hard is if you try fighting them with callahan or blitz/plague needle, and fail to see them sneak up on you.

Because of multiple Giant Lichen Colonies and now Compound 42.
Also, please don't necro old threads.