2020-04-17 : As you cast Back for More, it targets only a creature card in your graveyard. While the spell is resolving, you return the target card to the battlefield. When you do, the reflexive triggered ability triggers and you pick a target to fight. This is different from effects that say “If you do . . .” in that you can return a creature card to the battlefield without a legal target to fight.

2020-04-17 : If the target creature is an illegal target when the reflexive triggered ability tries to resolve, the ability doesn’t resolve. If the creature you put onto the battlefield is no longer on the battlefield or no longer a creature, the target creature won’t deal or be dealt damage.

2020-04-17 : The reflexive triggered ability from Back for More is put onto the stack at the same time as any other triggered abilities caused by the creature entering the battlefield.