I'm sure there will be a walkthrough for this. Me? Luckily I did know how :
While you're at the upper part of the road, when he charges at you which you should not wait for him, move Larry downwards so that the boss will throw a spear on you. Yeap, later you grab the spear and throw at him back and he's stunned and you know what to do.
Beware, the Level 3 freak monster could have been much worse. I've been there and it's probably the most annoying boss I ever faced.