You could always lower his saving throws and then cast multiple finger of deaths, or if you're playin a monk quiver palm the beastie, and as soon as you see its breath coming run away, and keep your casters at a safe distance, and i think wands of cloudkill also work, but be sure to use multiple charges as he heals quite often, incase you'd like to use the wand of cloudkill tactic just stay out of its LOS for ever, since cloudkill has a area wide effect, cast it somewhere near the red dragon, that'll surely help
