Hi EFTW,
If you took the FULL CAT (including the Essay and IR sections), in a realistic and test-like fashion, then you're remarkably close to your score goal right now. The 'tough' part is that, at this scoring level, the GMAT becomes really 'sensitive' to little mistakes - so if you make too many of them, you just won't be able to hit a 750+. With your Scaled Scores, you also don't have an obvious 'weak spot', so your missing points are likely due to procedural mistakes that you're making (not taking enough notes, doing work 'in your head', etc.). As such, you don't need to practice 'hard' questions, you need to fix the little mistakes that you're making. I suggest that you start with your last CAT and take a good look at every question that you got wrong - ask yourself WHY you got it wrong and what you could have done differently to get it correct. It might be that there are some 'easy' fixes to your situation; it's also possible that you'll need to invest in some new resources and learn some new tactics.
GMAT assassins aren't born, they're made,
Rich