Hard shmup is hard.
But seriously though, holy crap am I sucking at Ikaruga right now. Now, I have no problem getting through DoDonPachi on 4 credits on a MAME cabinet (which is much more impressive than it sounds, trust me), but I can't even get past the second level on Ikaruga. The second fucking level. So I have come to realize what the one most important factor is for determining success in a shmup.
I honestly hate to admit it, but the only reason I've gotten so comfortable with DoDonPachi and DonPachi is because, well, I play the shit out of them. It's the first game I play when i go to the retro gaming store *cough http://digitpress.com
that has a MAME cabinet, and I'll play it non-stop. Even when I'm getting my ass handed to me by it, I always want to come back for more. Well, now that I made myself sound like a complete masochist, I think I can bring this to a close.
So, to sum all this up, repetition is the only way to succeed in the world of shmups. So just be persistent, you'll beat that last boss eventually.
Here are the links. Make sure to read the text file inside.