Only level ive managed to get 40x combo on that gaunlet style level that just sends waves and waves of enemies...but when it comes to getting medals on all the other combat levels its bloody impossible...is there a technique to it? Same with that perfect knight combo?
One of the biggest tips for me is to use as many different moves as possible. Even if there's only one guy left and your combo is rolling, pull him in with the batclaw, stun him, jump over top of him and THEN strike him down. If you use all of Bats' moves during the round, you'll get 5000 extra per round.
Another thing I found is that when it says you have an Auto Takedown handy, you can Throw someone instead and vice versa, which helps boost your score. If you see a crowd or guys going for weapons, batarang them and then go for the nearest standing thug; if none are in sight, go for a ground takedown for some big points, though you have to be quick about it.
Follow the same motto on the extreme levels, only don't restart if you get hit. The massive number of enemies really boosts your score as long as you continuously build up your combos and keep use all of Bats moves.
I know the game is fast paced, but try and look around while you're in the middle of a long counter move/throw/auto takedown, and set up your next attack or get ready for your next counter.
Also, try avoid getting hit throughout the entire level for the 5000 Dark Knight bonus. It's near impossible on the extreme levels, but really helps for the first couple combat stages.
Practice, practice, practice and avoid button mashing and start to think about the round more tactically (It took a few playthroughs before I got my strategy for each round of the level) and you'll start to rack up big points.