Ghalta, Apex Devourer

This spell costs X less to cast, where X is the total power of
creatures you control.

Trample, hexproof

Whenever Ghalta attacks, you may have it fight up to one
target creature you don't control. If that creature dies, put
X +1/+1 counters on Ghalta, where X is that creature's
power.

Whenever a creature dies, you gain life equal to its power.