Sort:  

One mana card with up to 14 damage in one turn. For real?

I guess, only Exploding Dwarf can make more damage in one turn, but it only works in some rare situations.
Cursed Slimeball seems to be even more powerful when used with Crypt Mancer + Undead Priest, so all monsters have -2 HP. On the other hand, this card belongs to the Death splinter, the weakest one for maxed cards and Earth/Water with Prince Rennyn/Nymph or Valnamor/Mermaid Healer/Sea Genie have a lot of HP. It will make Death more competitive, but I think it won't make it an easy win for Death. Maybe it will just make people play less this boring Cocatrice + Brownie + Ooze line-ups.

I think 1 Melee damage for each monster would have been enough, the effect can not be avoided, right?

Yep, for me, it's a bit OP too. On the other hand, people will need to adapt to new rules, so less boring line-ups repeated 100x times.

Some time ago I was afraid about Fiendish Harpy being too OP. For some rulesets like Earthquake, Unprotected, Fog of War it is actually. It's a bit nerfed by introducing Furious Chicken because in most of the battles Harpy wastes 1-2 attacks for this Chicken. But.. Cursed Slimeball will remove the armor or even kill the Chicken so it would be useful to use both Cursed Slimeball + Fiendish Harpy (or any other monster with Opportunity).