Which monster is hardest to fight w/ chaingun?

As everyone else has the said, the AV. The chaingun's main benefit (aside from sniping) against mid-tier monsters is interrupting their attacks with the stream of bullets, but the AV has such a low pain chance it usually doesn't get interrupted. 

Someone more intelligent than me could probably work out the actual % likelihood of being able to stop an AV attack using the chaingun, based on the number of frames the AV attack lasts, the number of bullets the chaingun fires in that time, and the AV's pain-chance.

Arch-vile because it's not too vulnerable to pain, so you're quite unlikely to manage to make it flinch when it started incinerating you. With the SSG you can pelt it with 20 impacts at once, which increases the chance of flinching quite a lot. A monster like the revenant has a high pain chance so it's quite likely to flinch. Zombie hitscanners fall quickly to chaingun fire. Everything else can be attacked with circlestrafing so as to dodge their fire while filling them with lead. (Provided you've got enough ammo, even the boss monsters can be shot down this way.)

For me,it would be archvile,because he's fast and you need hide all the time from his fire. 

Any mid-tier monster if there's more than 1 of them in the room.

Spider Mastermind, because not only do you have to get out of its sight when it fires back, its pain chance makes it retaliate more often from the chaingun.

1 hour ago, Xyzzy01 said:

Spider Mastermind,

3 hours ago, NEANDERTHAL said:

Which non-boss monster

you are but a fool in man's shoes

I only read the thread title, oops :P

I retract my answer and say archvile, only because it takes so long to down one.

Probably, for me, the Archvile and Revenant. The Revenant is a pain in the ass to kill if you don't have the SSG.

Zombiemen

...

The Archvile please, but I never use the chaingun against him unless I don't have any other weapon. But the revenant does not lag behind, he is a pain in the ass with whatever weapon, for me personally. 

The zombieman makes it extremely difficult to go through all my bullets, so my vote is with him.

Close second goes to the baron for risk of death by boredom.