Strategy:Arch-vile

Arch-viles are the most dangerous monster seen in a standard game. They have a dangerous direct attack (a constant 20 fire damage) and the ability to resurrect lesser monsters in their line-of-sight. They typically spawn with a horde of such lesser monsters, particularly mancubi, revenants and formers. It is not uncommon for two or three arch-viles, with associated hordes, to spawn in the same room for extra fun. When they first start showing up, such a combined horde may be all the monsters on the level.

Since the damage and AI changes in 0.9.9.5, the direct damage from them is significantly less dangerous than their horde. Nevertheless, they need targeted as soon as possible, as they will (barring truly impressive firepower on your part) be able to resurrect the horde faster than you can eliminate them with your primary weapon. This tends to prevent using corner shooting, as the Arch-viles usually hang back out of range.

One good tactic is to use a BFG to damage the Arch-vile(s) and horde at once. This both kills them quickly, and tends to disperse them, reducing the amount of damage received. This is also a situation where a shockwave or Hellwave pack can be used to good effect.

If you don't have any of the above, a rocket launcher may be used to open up a passage behind the horde, for easy access to the Arch-viles.