Remember you need to plan out your game with team preview. If you see they have Volcarona, Blissey shouldn't have a big chunk from anything. As for handling Celebi and Latias, your own Latias handles the former unless they have Hidden Power Ice. Tyranitar shouldn't be switched into Celebi except on a free turn or a guaranteed Nasty Plot turn. It will gobble up Latias just fine, though.