This game is pretty easy. Just a few tips:
1) Start on easy or very easy. This way you can unlock all your moves and some nice accessories. But the best thing is: you unlock the immortal marionette accessorie.
2) Start a game on normal and buy/equip the imortal marionette.
3) Just start tapping the b and y button and jump around a bit. Normal mode completed.
4) Start a game on hard mode and repeat step 2 and 3. Collect some health potions for the final boss in the epilogue.
5) I havent started the hardest mode, but i suppose it should be a bit more challenging. If its too hard; replay some missions on (very) easy to earn halos and buy more health potions. O yea; you can get a bigger health bar by finding items ingame or buying it in shop.
6) I understand there are some glitches that can help you out as well.