Fern

Fern enters the battelfield with a +1/+1 counter
on it and as a copy of the strongest creature you
control except it is green, black and
non-legendary.
Copy any spell or ability you control 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 control, they fight.
Then put a-1/-1 counter on Firn.
If Firn would die, transform it instead.