Cepheus, Brilliant Gladiator

Convoke (Your creatures can help cast this spell.
Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Cepheus, Brilliant Gladiator enters the battlefield
with +X/+X, where X is the number of creatures
that were tapped to cast this spell.
Whenever Cepheus, Brilliant Gladiator attacks,
target creature gets +X/+X until end of turn.