Well, I don't know... I leveled up to around 70 (Squall), 62 (Riona) and 58 (Zell) and I managed to get past the Ultimecia/Griever combo, but I was defeated by the next incarnation of Ultimecia. She's appeared in more shapes and forms than Dr. Who

Mind you, I was fairly lucky. Odin's bigger brother (whatever his name is) appeared and took 9999 off Ultimecia and the GF together. Riona does know Wishing Star, and that took off around 45000 in HP. She even managed to use it twice, but I only saw Squall's Lion Heart once - though it did do alot of damage - around 5000 for a normal hit and 8000 to 9999 with the trigger pulled.