Step far from your opponent when your health meter is low. Then, hold 
High Attack + Low Attack. The health meter of your fighter will refill 
very quickly. Note: A full health meter is no guarantee of winning a match; 
a knockdown move against your character can still end the match quickly 
and your character can be attacked during the energy refill. 

Easy victory against the Joker (Master Challenge mode):
Use a knock down attack with any character at the start of each round.