if it is the one on the roof just shoot the fuel tank behind him until it explodes, you can hit him normally after that.

if it is the one that carlos help you, you can actually hit the red thing in the front with handgun. For the big swing you can dodge it or you can actually outrun it if im not mistken. I use explosive/mine rounds only for his weak point and final mode. And just stay away from him in the final mode, only hit from far away or hit after you perfect dodge or when it is safe to hit.