In extra challenge rooms, it seems like once lichens merge, they are no longer counted as 'living'. This means that if you don't kill the lichens yourself, but kill all of the other monsters, the next wave will spawn even though there are monsters still on the field. Normally this wouldn't be too huge of an issue, except that occasionally (all the time for me) lichens manage to form a colony before I can deal with them. Usually I have to try and clear the junk first, then the pseudo-boss monster.

Which means that when I accidentally kill the last jelly cube, there is suddenly an additional 10 jelly cubes and six lichens... In addition to the mid-size lichen colony that's about to grow horns and eat me for lunch.

I'm not sure if this is by design, but if it isn't, could you make it so that the next wave of monsters doesn't spawn until all creatures in the challenge room or dead? As it is I think it's more of a challenge than is really justified.