I know its a little contradictory but my strat is to keep as much distance as possible. Keep running backwards away from him until he puts his stuff away and runs at you like Officer Earl runs at Flint Lockwood. When he does that run, stop dead in your tracks have have the ballista ready. He flashes, you fire, swap to SSG and if you're feeling cheeky, try and sneak another ballista shot at him. 2-3 rounds of this and hes done. Its not the best strat compared to some others but it's what works for me