The best moves to use are upper cut, round house, low leg kick. The low leg kick is useful for keeping your opponent out of range and on target for a round house followed by an upper cut. The double backfist is also very powerful and often provides a knockout punch.
If you are losing a fight, and don't have much chance of recovery, then press the 'ESC' key so that your stats remain intact.
Efficient boxer:
A - 34
B - 05
C - 17
D - 11
E - 08
F - 32
G - 04
H - 33
I - 01
J - 55
K - 10
L - 25
M - 16