Borrowed wholesale from salut allo's 2007 solution for Ace. I used this on the first level and unlocked Untouchable and Diplomat on my first try.Solo solution with two controllers and two accounts.
Going Under the Screen
This only works between local players.
This is a glitch I found out years ago, and I think there are other solutions about it, but I'll still write it in here.
Using 2 controllers, you can go under the screen, making you invincible to everything. To do that, plug in two controllers. Take the controller with your main account (one that needs ace) in your right hand, and the other controller in your left hand.
Right when the level starts, you will want to bring both ship at the bottom of the screen, and hit cn_X on the controller in your right hand, then wait about half a second and press cn_X again on the same controller. Do the exact same with the left controller, then repeat with the right controller again, and as you do this, you should see your ships slowly going under the screen.
(To make sure you are completely under the screen, do it around 2 more times when both ships are out of sight.)