Maybe we can give you some advice on how to use them with tactics.
2x lv120 unit enough which are Yuna and Tidus
Sinple really, used xiza bell on xmas viktora to dispel spread once before, and then after the heads were dead, then sakura limit for 3 chain, then yuna limit break while using a xiza to steal time
More generally any aoe that connects with the body will hit all the heads, even lucia split shot will hit all the heads
He's pretty much unbeatable UNLESS you use a lot of AOE attacks; you absolutely have to eliminate the individual snakes before you can defeat his main body because they heal him so quickly.
I am going to assume that you are referring to the battle in the story campaign where you take Muraga on and base my answer off this assumption.
This battle being a part of story progress is just a checkpoint for growing accounts. I recommend investing in a diverse team that can handle story content for you. What I mean by this is: Pick a Vanguard Tank main job unit, a mix of 3 Spellcaster/Striker/Attacker main job units, and a Healer main job unit. Once you have this party, invest in them to get their character shards for limit breaking and materials for awakening them fully.
Having your own sturdy and well rounded team composition will allow you to handle story content and event quests of "Hard" difficulty(typically recommended lvl 60) in the future.