Not all people rely on Squall and Lionheart. After all the chances of getting Lionheart aren't 100% either.
But people do evenly balance every character, so they can make light work of Ulti. Take me for example, I've 100%ed the Euro Game [spoiler](Can't 100% completely, cos of Chocobo World)[/spoiler] but still, all GFs and Characters are Level 100. All my characters have a GF with Str-J and HP-J, which are, in my opinion, the 2 key Junction Abilities to defeat Ulti. High HP = Low damage taken in, High Str = High Damage given out. Bearing this in mind, I do have my own little "key party", as I call it, which consists of Squall, Rinoa and Selphie. But with GFs equipped to all Party members, I can beat the game with Quistis, Zell and Irvine. I like to call it Consistance Training. If you're unsure with what to do on the 4th Disc, head to the Island Closest to Heaven/Hell and give Consistance Training a go. If it doesn't work, then you might not have done it right. Try getting Ultimate Weapons, changing your strategy for defeating monsters (mine was 100 Death Junctioned to ST-ATK), use anything you can. Even, if need be, try Kamikaze. Kamikaze is an underestimated maneuver. Sure, it annihilates one party member, but the move could become your best friend. It delivers around the same amount of damage as Eden. Try experimenting with different moves and abilities. If you still can't beat the game, then you're obviously doing something wrong, so change strategies again. It's the changing factors, that makes this game so much better than any other FF.