Wakka is the strongest member you have in your party because he has the best overdrives in game: Attack Reels.
Attack Reels : 12 hit
Blitz Ace : 8 hit
Also, if you can completed the entire sphere grid with Wakka and Tidus, your Attack Reels and Blitz Ace will most likely do 99999 damage every hit.
As for your Question:
Yes, I used all party in battle even normal/boss battle.
Auron is slow, his celestial weapon, Masamune can't executed 99999 damage unless you obtained 255 strength, Masamune can execute 99999 damage if Auron is in critical state.
Originally, most people use Wakka and Tidus to beat monster arena because they are brilliant character.
My Wakka, Tidus and Yuna can already succeed 99999 damage with their physical attack and Quick Hit.
Yuna is the important character in party, reason:
A) Nirvana allow her to use any magic/skill at 1 MP cost
B) Her Stat determined Aeons Stat.
I increase Yuna strength stat so greatly, that my Valefor already have 255 strength.
This three person is best to used to fight Dark Aeons.
I'm currently, training to boost Lulu, Rikku and Kimahri stat.
Rikku: Important against Shinryu
Lulu: Magic Booster and One MP Cost is an important thing for defeating monster who are immune against physical attack like Jumbo Flan.
Kimahri: Don't know what the use of him, but he is my favourite character besides Lulu in FFX.
Hope that help.
