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.

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.

You don't choose a target creature you don't control at the time you cast Back for More. Rather, a "reflexive" ability triggers when you return the target creature card from your graveyard to the battlefield. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal. Notably, Back for More isn't a crime, but the reflexive triggered ability potentially is.