The lava tiles I've been making only do 8 damage if you start your turn on them. Granted this is early in the campaign, but I don't see why lava terrain would grow deadlier later in the game, or in multiplayer.
Hmmm they changed it up. either that or I remembered incorrectly.
Lava used to do 10 damage but it was reduced to 8. I've been in the game since the early beta and I'm pretty sure it's never been 20.
You
could do 20 damage to a big 2x2 size enemy if you cast two lava tiles under it but even that was changed so the big enemies only take damage from one lava tile per turn.
There is technically one card in the game that does 20 damage (
http://wiki.cardhuntria.com/wiki/Cards/Almighty_Bludgeon) but that isn't on any currently available items. It's probably reserved for future expansions or something.