Make sure to rotate your enmity combo between the mobs to keep aggro. The enmity metre is on the party list icons, full bar is full aggro. So make sure you keep your enmity up on all targets.

Breaking into tanking you can number the targets to tell the DPS to attack 1, 2, 3 etc. You still must keep aggro on all targets or you will have mobs running after Arcanists/SMNs(AOE DOT damage) and your healers with heal aggro.