DMC 2 help

I've beat Lucia's disc numerous times...i just keep playing that one to level up the weapons and get better items. but im stuck on Dante's disc. I dont remember which mission it is, but the one with the helicopter. I always just run to the top a fight it there, as the FAQ's says to do. But i can never beat the thing! I'll get its health down pretty low, and then it just kills me! I gave up after trying the mission 10+ times. Any tips?

*Edit* This is the guide i've been going by...its mission 5.
http://guides.ign.com/guides/482048/page_5.html

I got this problem too, Just go back to mission 1 and get enough red orbs to buy a hell load of blue and purple orbs.