If it's already enraged, forget all of the above and fill it with lead.
lol no.
as long as it gets engaged far enough away (by dumbass demos probably) you don't want to spam it. the rest of the team should call the target to the sniper and then return to kiling off lesser specemines. for the sniper, line up and wait for the right shot. don't just spam at it, because you might miss. this is also true of fp's. the best time to hit a fp is when it begins to rage. it's standing still, the head is framed by it's arms, and it's just begging to die.