Help with brink of extinction boss battle

Last quest I need to get my quest cape back and I can't beat the boss :/. I can get him down to 1/10 health in a minute or so but when I do that it just brings up all 4 of the extra monsters instantly so then I have the bosses occasional 1k damage and then 4 rangers doing 500-1000 damage each every few seconds.

I was using a sara sword but then I just switched to chaotic rapier and a crystal shield to try and absorb some of the ranged damage. Still damaged the boss a ton but the ranged damage didn't go down at all :/. Mm and for some reason youtube videos won't play for me so no I can't watch a bunch of guide videos to do it.