ZDoom

Spoiler:

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When you get to the battle area with the mirrors and invisible pinkys, it's a cool fight that's tricky but doable, but then having an invisible archvile pop out WITH tons of other enemies, including hitscanners (Namely the heavy machine gunners) is a major pain. You are asking the player to essentially slow down if not stop next to a mirror to scan quickly for the archvile WHILE approaching imps,pinkys, shotgunners and most annoyingly machinegunners are approaching and tearing you apart, it's too much. I would suggest two things: One, make the Archvile visible, no need to have it be invisible with so many monsters crowding around you, just make it visible, or half-visible so keen players can still spot it.

Second suggestion is to get rid of the heavy machine-gunners, possibly the shotgunenrs too, at least dealing with the imps and pinkys is tolerable if annoying, but it's not so bad to deal with visible demons as you hide behind the mirror pillars so you can get a breather in to hit the archvile with some heavy firepower, IF you even have any at that point.