It is possible to stun him and kill him in ancient rage. So any crowd control ability can pin him allowing you to either move away or let the team focus damage on him.
If you are in a area with good cover you can flank him with rocket boots. A few close shots send him into critical. This is especially useful If he has attacked some one else with plant.
use cover to get close and then use runic blast, shield recharge(if his shields are up), lock on attack(until he starts shield melting you). then move to other targets and into cover