Snapjaw is the nemesis for characters that rely on passive abilities like Bleed. By the end of the seventh turn, every hero will be dead (56 damages), unless he has some healing potion. The plant must be killed so very quickly.
Hydra, on the other hand, is even harder, as you have to deal an average of 20 damages per turn. Piercing comes in handy, as any other ability that lets roll multiple dice for damage.
The special achievement looks pretty futile because, if you did not bring the beast down during the first three turns, it is unlikely that you are doing it afterwards, having already spent a few of your abilities.
I never, ever, defeated either Kindle or Magmageddon. I think I should try the "nerfed" version, next time I play.