You get a point each time a unit hits for more than the advertised amount, in this case 50,000. That's why 2x G3, another heavy hitter, and three light hitters [penguin] are good, so you can build up the chain/activate G3 without killing the mobs [that's where penguin come in] and then get a point for each of your three powerful units [assuming you activate G3 and don't miss a Perfect on the penguin].
To compare, my team was:
G3/G3
Crocodile [Slasher]
3x Penguin
You just need to get a score of 25 to get all of the rewards for this current ranking.