So many other ways this could have been templated, but I think it falls under the umbrella of not having proper targeting rules. If the intention is to fight with a single creature (pretty obvious to my eyes), then you should have to choose/select/target (whatever keyword or nomenclature) that creature at the outset of the effect.
I guess the simpler way to clarify this issue is to say that the indefinite article "a" means "one," but then we may as well be re-hashing the "does may mean may or does may mean must?" controversy of years past. English is hard.
Edited by WonderWAAAGH