Your aeons against mine!
Grothia (Fist in the PS3 version of Final Fantasy X HD Remaster) is the name Isaaru has given to his aeon Ifrit. He is summoned to fight against Yuna in a battle of aeons when the party is trying to escape Via Purifico in Bevelle.
The best bet is to use Bahamut; however, he will be more useful against the more powerful Pterya coming up next. Out of those remaining, the one with a clear advantage is Shiva, summoned with an Overdrive at the ready if possible.
The added Ice effect of Diamond Dust should be enough to destroy Grothia straight-away, or leave him badly wounded. Casting NulBlaze on Shiva before Grothia can use its Overdrive will negate the effect of Grothia's Hellfire.
Should Shiva fail, summoning Valefor is the next best option, as Grothia's normal attack has a high chance of missing its mark.
At this point in the game, all aeons' Overdrives should be dealing damage of 9999 and since Grothia's HP is only 8000 a single aeon Overdrive can defeat him.
Grothia is Greek for "punch" or "fist". Likewise, Grothia's original name, "Kobushi", is Japanese for "fist".