So, Elements already has instakill mechanics.
Freeze+Shockwave: Two card combo
Mutation: 10% chance
Maxwell's Demon, Otyugh: Conditional
What if there was an instakill card with a steep drawback? For example:
> Kill target creature. Damage your HP equal to the target's quanta cost, attack, and HP.
So killing a new Lava Golem would be 11 damage to you, +4 every time it's grown. So at 7 HP, when it's out of the range of one Lightning or Rage, you'd take 23 damage (25 upped)
Armagio in your way, and you need it gone sooner? Kill it, take 31 damage (37 upped). Playing vs a RoL/Hope, and you can handle the Rays, but not the Dragons? 32 damage (also 37 upped). Nymph Queen threating to swarm you to death? 19 damage (21 upped).
Surely this concept can be balanced.
Someone else provided this example:
> Kill target creature. You lose the game in X turns.