The second hardest of the Naruto online achievements, Zabuza is slow and cumbersome. But like Orochimaru, he has strengths.
Down+Y, A+X then a perfect Level 3 Water Dragon jutsu leaves them with virtually no health. He also has incredible range, Back+X, Back+Y being very good moves. Remember, the key to dominating online is not to try to do full combos or the same move over and over again, its completely mixing up your moves and doing half combos, pulling off jutsus when you can etc. It leaves the opponent in a position where it's hard to guess what you'll do next. If you do get the chance though, a good way to eat your opponents health is to do a Level 3 Chakra Concentration and then use Forward+Y,Y,Y,Y,X. Does nearly half damage! Remember, practise is the only way to really get these achievements, practice against level 1-5 (moving up chromatically) CPU opponents if you're really stuck.