The missions within the 352nd F.G. will be your toughest challenge yet. Not only are the bonus objectives B.S., but the primary objectives, enemy planes shooting you up, and the fact that your health does not regenerate, you'll restart at certain checkpoints for some time.
The aircraft you unlock for completing those missions on Ace include the "Ace" Fw-190, Meteor, and the "Ace" Me-262.
I did it too. The 2nd 352 F.G. Mission's bonus objective was **** because of the fact that about a half/second burst from ONE of the Focke-Wulfs takes down the "Betsy". What I did was beat the last level and use the Ace 262 for this level, and even then it was close. Congrats to anyone who can do it, too... I could use some tips for that... and BTW the P-51H is the bird you'll probably be using for most of Ace Mode.