I'm not great, only got 3 batarangs on 4 of the challenges, 2 on the rest, but the key is combo and combo variation. People say you should aim for the dark knight reward (take no hits in any round) but i say that should be something to not worry about too much, by all means try for it but prioritise other forms of point collection above it.
The main way of collecting points is combos and combo variation, each attack is multiplied by your combo count, so at a combo of 20 you'll get 20x10 points = 200, but then certain moves net more points, an instant takedown for example is 50 points (so 20x50=1000) and a ground + pound (crouch + triangle when an opponent is down) is a whopping 100 points (20x100=2000, that's 2000 points for one attack!). Always ease your way into a combo with a few critical attacks (time your punches correctly for double the combo bonus) and as your opponents are beginning to get knocked down start unleashing the big moves such as instant takedowns and ground and pounds. While they net the most points, they also knock out an opponent for good so you can't get anything else from them again so save those moves until your combo count is higher and then use them for the maximum points possible.
Never underestimate the grapple (double tap R2) and batarang (tap R1), they don't add to your combo but they DO slow down the enemy and they DO add to your variation bonus at the end of the round. For example, say you used punch, ground and pound, instant takedown and counter in a combo chain, that's 4 moves, a variation bonus of 4 moves gives you 500 points at the end of the round. Now say you added grapple and batarang to that combo, two weak quick moves, you'll then get a 6 move variation bonus which is a whopping 2000 points. Add stun and jump over (double tap x when near to an enemy) and that gives you a bonus of 8 which is 4000 points iirc! Get full variation into each round and that's effectively a free 16,000 points basically, not even taking into account the potentially huge amount of points you will have got from your instant takedowns and ground+pounds.