Destroying the targeted monster and inflicting damage are considered to happen simultaneously.
When resolving this effect, if the targeted monster is in face-down Defense Position, the effect is not applied. (The targeted monster is not destroyed, and no damage is inflicted to your opponent.)
You can target an Xyz Monster that has no Level with the effect of "Genex Ally Volcannon". The targeted monster is destroyed, but since it has no Level, no damage is inflicted.