Most Difficult monsters to deal with.

[Unknown_Assassin]

Incarnate (or known as Death Incarnate): It recovers health by using hitscan attacks on you.

It does? Since when? O_o In either case, the DI was meant to be either be killed by one of EP3's Always-X-death weapon, or the EP3 ice weapon. Using it with normal doom weapons would be ludicrous.

Oh....oh...ooooh. I must have mixed that ability up with your Soul Leech weapon for some apparent reason. o_O. I've been using that weapon way too much.

I do know how to stop the Incarnate, but if it's used in levels that don't have your weapons, then the monster is quite difficult to handle. If I remember correctly, it was used in a wad that contain only normal doom weapons (UTNT or maybe ZPack?).

[Ethril]

The Death Incarnate, huh? Yeah, those can be pretty nasty. Let's see... Ice death, XDeath, Morphing it and then killing it... Does it stay dead if you Crush it, or does it get up anyway, only now it's a "ghost"? I'm pretty sure you can stop it from reviving if you stand on its corpse while it tries to get up, too.
Also I've noticed that sometimes it doesn't revive even if it dies normally and isn't obstructed from reviving. Or maybe that's just the AEoD version.

[NovaDragon]

If you crush it, and all the other methods mentioned, you can permenantly kill them. Im not sure what the percentage is but there is a fair possibility that if you kill one, it just wont get up. Thats in any version as well, but if i recall the chance of it happening is probably only 30 or 40%. You would be best off crushing them in a door if possible, thats what i did everytime in scoredoom.

Zpack has them and Demon Eclipse im sure may have them in the future. Otherwise zpack is the only level set so far thats included them normally. That was also in just one level and it was 3 of them.

[SoulCrow]

They originated from Demon Eclipse, and they are included in the Ultimate Torment and Torture as well, to my knowledge.

EDIT: ...and City of the Damned: Apocalypse

[Amuscaria]

The newest version of the DI cant be stopped from reviving simply by standing on it, as it loops it's respawn-check sequence continuously until it's respawned. They can be crushed, however. Although they are largely planned to be used in large relatively open areas to keep them from spamming their shot-gun like eye-attack in a small room. These areas, wont have doors or other crushers around.