Our guild does everyone but 1 person stack on the boss, usually a hunter stays out, lays traps as far back as the healers can reach, and then all the ranged (3 on 10man, and about 10 on 25man give or take a few) turn around and kill the parasites, use everything you got, shadowfury, traps, frost novas, etc etc, to slow and stun them while the hunter(s) kite them, and pew pew (single target if necessary, whatever gets the job done). When he does mangle he doesnt always spawn another pillar, so while people are getting the head down the ranged have time to finish it off before his double damage phase. Has worked pretty well for us, hope it helps
Edit: Tank tends to stand on the left, healers and everyone else on the right edge, and we 2 tank incase the tank dies during mangle