To beat the heavy gunman, first take out the four snipers on the two metal walkways and then the two on the ground, then run counter-clockwise in a wide circle shooting all the time. Only stop if your hit points are getting low to hit. It's best to have a phoenix down already equipped also. Use the Griffon 2 with a long barrel, and have on the Cerebus Relief for a slight extra defense boost. I didn't bother to use fire materia magic.

Hope It Helps!