equip someone who has 9999 hp with two materia and dont give the other two too many either. do knights of the round and mime it a couple of times at first. at this point i find it usually puts its little lights on. just endure this and keep the two materia persons health full with full cure or whatever. usually after this theres a tam-air storm which will take anyone with 9999 hp and two materia down to 7777 hp, hence lucky 7s. This is largely good thing and takes quite a bit of hp off but will stop eventually. another two or three KotRs should finish the lad off though.