To me, the big benefit of immolation isn't that it makes the guards not as dangerous (I agree that they're far from the biggest danger of V5), but that is stops them from blocking line of fire, gives you a giant wall of fire clouds, and means that you don't take a few turns of guard attacks if you try to retreat up the stairs. It attacks a lot of enemies, but they don't all show up at the same time, and between having clear line of fire and any enemies trying to get into melee range having to pass through 3-5 fire clouds on the way, it's usually pretty easy to kill enemies as they arrive. Eventually their numbers might build up, but fortunately, you're already standing on the stairs, so you can just leave as long as you make Vault Wardens high priority and don't let any constricting enemies reach you.
Sar wrote:I used to do that and I liked it, except I would go into a corridor first and explode the guards second. The noise would attract monsters to the centre, but I would be at the rim by that time.
I like quaffing hasting and blinking into a corridor before exploding the guards if I have good ranged attacks and not good fire resistance. You can't retreat up the stairs, but enemies will only come from two directions instead of all of them. Most of the enemies attracted to the noise will come from the center, where the fire clouds will damage them, and with this strategy you can position yourself so that the fire clouds are at the edge of LoS, so even ranged enemies will have to go through them (while staying at the stairs means ranged enemies can fire at you from the other side of the clouds). Some enemies will come from behind too, but usually not too many, from my experience, and anyone doing Vaults 5 should be more than capable of handling them.
Also, if you're worshipping Qazlal, it's really fun to do immolaton -> disaster area -> elemental force. Tons of explosions, everything on screen dies, and you end up with 10 or so fire elementals.