Stratoavis has 320,000 HP. You're characters obviously aren't strong enough so build up their stats a bit more.

Alternatively you can summon Yojimbo, keep paying him whatever amount you want, and have him attack for you(I am not talking about Zanmato here). Due to Yojimbo's high evasion, Stratoavis will always miss. This method is slow but almost guaranteed.