He has good burst damage, and he's fairly decent at skirmishing. I've found my standard combo with him, it seems to be the most damage effective one I've used so far.
Skill priority R > W > E > Q
Start in cannon form. E between you and him to initiate, fire a Q at him if you have a clear shot while running at him through the E, activate W when you get in range and fire off all three shots. Transform. Q to him, W, auto attack, E to finish. Transform back to cannon shortly after your W runs out. Your E should be back up, along with W. Repeat. I've found that most people won't live long enough to reach the hammer > cannon transform.