Once you get Bahamut and the ability to make Trio of 9999, FFX's normal bosses are way too easy.
Grand Summon Bahamut, Mega Flare, Mega Flare, dismiss, Trio of 9999, Blitz Ace/Whatever Tidus's second move is called. Bring in Auron and Wakka to clean up.
I've still never managed to beat Ballos in Cave Story, namely because I can't make it through Hell.