Killing the ace is easy even on Insane. On Insane, you wingmen are a lot smarter and will kill the enemy easy. The key to winning before you have shields is to lob all your missiles at the enemy and keep calling the support ship. The AI sucks at dodging missiles so you shouldn't have a hard time killing the Ace (I completed the mission this way loosing only one wingmen).
The ace only shows up when, 1, you win the first mission (Eve of Destruction, guard the Orff, super-easy), and 2, two out of the three other enemy wings have been destroyed. So you have to pay attention.