My favorite way to tackle these rooms requires a little prep:

1. Stand back from the door and throw a random object to keep it open
2. Step on a swiftthistle or otherwise activate hourglass trinket to freeze time
3. Toss a potion of paralysis into the centre of the room, immediately followed by a potion of toxic gas and then a a liquid flame for good measure
4. Wait for time to come back and watch the damage meters/kill messages go brrrrrrr.

If all goes well, you should only have one of two stragglers left at the end to finish off, if at all.