Firn

Fern enters the battelfield with a +1/+1 counter
on it and as a copy of the strongest creature you
controle except it is green, black and
non-legendary.
Copy any spell or ability you controle that would
affect your strongest creature. Fern is the target
of the copies.
At the beginning of your endstep, if Firn isn't
the strongest creature you controle, they fight.
Then put a -1/-1 counter on Firn.
If Firn would die, transform it instead.