My guess is tank taunts one, healer taunts one and then the dps kill them one at a time.
I have done this achievement many times, for multiple groups, and I did it quite differently than the above poster.
If you imagine the original boss position as best as I can draw:
Players come from this direction
The numbered are the bosses, the @ is the big giant pillar the three stack around. When I start the fight, I taunt 3 (the melee add) and take him all the way past the pillar, down the slope, to the (X); I'm basically standing one foot in the water, one on the island. At the base of the slope, if you have your other 3 players stack at (Y and Z), on the opposite side of the pillars, they are out of line of sight for the leap. (Tank will not receive any healing for duration of fights obvs)
DPS just burn down 1+2 quickly, then you drag 3 around making sure you never actually step in the circles they leave after they die.
EDIT: Making my island look like an island