Is there a way to win? As far as I can tell, I seem to have gotten everything, but I can't figure out what to do next. (Also, the only way I could get to the giant is by damage boosting; is there some ability I'm missing somehow?)
There's no explicit victory condition; feel free to consider yourself to have won if you've beaten all five bosses (as indicated by the orbs in the lair). Exploring the whole map, amassing a hoard that you're proud of, or just deciding that you're done are all also valid ways to win.
There's a couple ways to reach the giant; rocket jump + wings + lunging bite is my suggested damage-free approach.