I selected Felipe Cruz and spammed his
attack and pushed the opponent into the corner for the 20 hit combo, 250 and 500 Damage Achievements.
Then just use the
attack and your LT special once you accumulate it. Don't worry about the enemies special you just take it and go back to smashing his skull in.
You should get 800g in about 2-3 minutes as you have to win 3 rounds.
A guide if you really need one is below.