23 Jun 2020
You can cast Primal Might with 0 as the value of X to just have the two target creatures fight.
23 Jun 2020
If the creature you control is an illegal target as Primal Might tries to resolve, no creature gets +X/+X. If that creature is a legal target but the creature you don't control isn't, it still gets +X/+X.
23 Jun 2020
If you choose two target creatures and either target is an illegal target as Primal Might tries to resolve, neither creature will deal or be dealt damage.
23 Jun 2020
You can cast Primal Might targeting only the creature you control.