Golden Sun: The Lost Age Tips

Easy experience
Go to the Mars Lighthouse and look for Firebirds. One Firebird can give you as much experience as the battle with Saturos and Menardi (6,000 experience points). You can also look for some Phoenix, which can give you about 3000 experience points. Because they have low HP, this is the easiest way to gain experience.

To get 17,000 experience points, go to the few long hallways before fighting Sentinel. You should encounter a blue bird that gives 9,833 experience points each.

[Moderator Note] You'll have to be quick. The birds like to attack two times per turn and flee.