Well,Aeris wouldn't do that,but......assuming she did , it would be depending on what they have (i.e,materia,weapons,limit breaks,ect...) Aeris could have a chance,because of her healing limit breaks,but her defensive & Attack abilities are quite low.Tifa,on the other hand,lacks healing limit breaks,BUT she can hit Aeris multiple times using her limit.Tifa has the upper hand (assuming they are not using materia & just attack/limit break)