Seriously way too easy.
At depth 16, I was able to solo yet another. The only reason I died is that this was the first time I encounters the "super mega hyper combo giant lichen go!" creature -- heck, the first time I've seen lichen combine at all.
After taking it out, and killing the rest of the level, I think I had 15 hearts lying around that I couldn't take with me.
And this is solo, with just a one-star sword and (... Blaster is a two-star gun, right?) Blaster.
Please, do something to make it tougher. Some of the sleeping lichens, or something. But plain yellow ooze on the ground that just wipes up with some sword swings? And so many heart drops?
The deadliest thing on that level is (A) Lag, and (B) Gun puppies appearing out of no where and hitting you while you are lagging :-).
Some variants of the lichen levels are also a bit short... comparing lichenous lair to the Vanaduke fortress levels, it's almost astonishing how much variation there is in what you get for 10 energy.
That said, one thing that could make this harder would be to increase the likelihood that two lichen will merge. I've taken to blasting away all obstacles, then sitting still for a minute or two in hopes that a giant uber-lichen would form and make the fight more interesting. Otherwise, you're faced with a small army of small, fragile yellow things that can be destroyed with a gun long before they're ever close enough to threaten you.
Alternate suggestions: allow two large lichen to merge; reduce the drop rate for heart containers, more gun puppies or fast enemies (to prevent you from standing still and firing a gun without risk); seed the level with some pre-merged lichens to speed the formation of the uber spikey fellows. (who, incidentally, are still slow and short ranged enough to be taken out with charged gun shots and low risk)