From The FAQ, which I can send you if you would like.
Necromancer & Black Tiger covered this, but just in case.
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Boss Battle: Guruda
Recommended Level: 10
Guruda flies back and forth through its domain, attempting to slam into you.
Couple this with a projectile attack and you have the hardest boss thus far.
Only Guruda's underside can be damaged, so stay on the ground floor as much as
possible, jumping up only to dodge a charge or volley of feathers. Use Fire
Blast (Level 2) to hit him from a distance and avoid having to get in close.
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I would always stand at the beginning of the level for half an hour or so, and wait for the forest monsters to keep coming, racking up a few levels by walking back & forth. Even though the FAQ says level 10, I wouldn't fight him until level 15 or so.