I'll just mention the ones worth mentioning, the ones I don't mention are trivial

Here is a list from most difficult to the least:

Grievers - These can easily wipe a group if not handled properly. Shock Grievers and Freeze Grievers are the worse as they can outrun and immobilize a knight. The other grievers aren't as bad as you can still function after getting hit. The order of difficulty is Shock > Freeze > Stun > Fire > Poison. In a regular floor, they should be freed and pulled separately if possible because in a big group, there is no way to properly interrupt them all. In an arena, they should simply be zerged down, even if one or two people have to sacrifice themselves to do it, as leaving them up would result in an eventual death for the entire party anyway.

Slimes - While Lichens aren't that bad and can easily be handled when merged, their counter parts can pose a huge problem. The hardest one in the family is probably Quicksilver. These will become immune to damage and move at an incredible speed, killing everything including other mobs, knights, and even themselves. The Toxilargos are the next on difficulty as their piercing spin will decimate a party when they are amassed in a danger room. And finally, Oilers are fairly easy to deal with unless they are grouped with other mobs or idiotic knights who light them on fire. When lit, Oilers become harder than even Quicksilvers.

Gremlins - I will only mention t3 as the other versions are trivial. T3 gremlins are annoying as they can only be attacked from the front and are usually grouped up. While not hard, their numbers and defenses makes it frustrating to dispatch.

Wolvers - Like gremlins, they are generally grouped up as well and are frustrating to deal with even though they are not hard.

To fight Devilites:

Yes-Men cannot attack. Pit Bosses also cannot attack.
Pit Bosses can either give a promotion to a little Devilite (turning it into a frenzied, furious and hostile Overtimer), or give it a promotion (turning it into a Yes-Man).
If a Pit Boss dies, another Devilite in that room will transform into a Pit Boss and any nearby Yes-Men will turn into hostile and angry Overtimers.
The solution is to kill the little ones first, keep the Pit Bosses away from them with shield-shoving or knockbacks. Killing the Pit Bosses first is an exercise in futility, as a regular Devilite will take his place each time you kill one. Killing the Yes-men first is unhelpful because they, like the Pit Bosses, are incapable of hurting you. Get the hostile Devilites first, then get the Yes-men second, then get the Pit Bosses last! And hope your teammates don't assume "bigger = dangerous," because killing the Pit Bosses first is counterproductive. Doing so means you're punching at an enemy with a bigger lifebar, with the result of removing a REGULAR, WEAKER, SMALL Devilite from the field in the end. Just go for the

The heal-moth (Silkwing) is actually a Fiend-type creature. It's weak to Piercing. A Flourish will dispatch it merrily. (Flourish will dispatch Devilites merrily too, given that you can't really hit them too well with guns.)

If you have bombs, you can plant one in front of a Trojan, then circle behind him so that his back is facing the bomb. If you have a teammate, just remember that he can only face one of you at a time-- the other guy has to slip behind him and bash the weakpoint. (Preferably with a Piercing weapon-- hello again, Flourish!-- but the Calibur's charged attack will delightfully shred him too.)

The "Spectre" (Phantom) can pass through walls, but his charged attack can't. You can duck behind a headstone to be safe from those big projectiles. (In Tier 3, he fires three of them in a spread so you can't just sidestep it.) They are susceptible to Stun and Freeze-- with a Freezing Atomizer or Shivermist, you can re-freeze them and keep them locked down indefinitely while the rest of the team runs forward to clear out the zombies for good crowns and heat. (GREAT crowns and heat, if it's the second half of that tier.)

Wolvers are weak to Piercing, but due to the vast majority of those weapons being projectile-based, they auto-dodge all piercing attacks that aren't the Flourish (I like this thing). When an Alpha is getting ready to bite down on you, don't block unless you're using a Plate shield! They can chomp right through anything else! Just run directly away from them, and charge up your counterattack. Do not stop running until after the third bite has come out, and missed. THEN punish them.

Tier 3 Gremlin Menders can shield themselves just like a player can, and revive other defeated Gremlins. Just in case you didn't hate them enough already.

Retrodes (laser androids) have surprisingly good tracking and long reach on their melee attack. Don't bother trying to dodge it-- just block it instead.

In order of difficulty for me (with Heavy demo helm and suit and wise owlite shield with UV low shadow for defense - so I'm short on shadow protection):

Greavers are the hardest for me. If I can get them to attack me one at a time, I tend to try to play paddle-ball with them with my flamberge (hit them with the first swing to push them back a little, hesitate so I don't lunge forward, repeat). If they come in a group, shield bump, flail wildly, shield bump until they are dead or I am. If I'm really lucky, I'll have time to put a freezing atomizer between me and them (soon to be shivermist), and then I have a teammate use a hail driver while I continue to lay down the freezing mist. That works really if you see them far enough away, well but it isn't an option once they're in your hair.

I only recently discovered (by burning to death repeatedly with no apparent cause), that tier 3 greavers drop a status-inducing mist in addition to biting your face and shooting 4 status-inducing balls of death. That mist can be hard to see, so don't run around willy nilly with greavers in a tier 3 room.

Devilites are also tough, but only in large groups, especially in spike-filled rooms. Lots of overtimers means pitchforks flying everywhere, and those pitchforks have a long range and induce status effects, so kill overtimers first. Freezing bombs help a lot, but you'll need a CTR UV or some demo gear if you hope to be able to lay them down before getting forked. Flourish line swords also help, because devilites are weak to piercing. Guns are darn near useless unless someone else in your party is laying down freeze mist to hold them still. Even then, I would stick to hail drivers for the ricochets and refreezes.

Alpha wolvers have amazing range and do huge damage with their triple bite. Freeze them in place. Just blanket the room in freeze mist and the triple bit is totally neutralized because they can't move forward to get you. The teleporting is unusual, but it seems like they can only teleport once after being frozen, and they always emerge from that port frozen in place (even if outside of the freeze mist) and ready for a sword-wielding teammate to beat up on.

Phantoms are only bad for me in tier 3 and they're particularly horrible with a party of 3 or 4. Freeze bombs and hail drivers are great, a shield with shadow defense is a good idea, and don't forget to lure them into a room you've already cleared of zombies.

Gremlins are difficult, but only if there are two or more menders around. 3 menders is terrifying in tier 3 because it takes so long to burst their bubble of invincibility and they can resurrect each other. Kill the menders first, freeze if needed to control the crowd, fire bomb to deal damage to large groups (especially the annoying darting demos) and try to separate them one at a time and finish them off with swords.

In conclusion, I need more shadow defense in my armor.