Don't forget to combine your ogcd heals with a shield. As soon as the shield applies, because this briefly stops damage and tops the tank without their hp continuing to drop.

Wait until their hp is low enough about half, or based on the rate at which thier hp is dropping, as to avoid over healing. Spreading your mitigation is important. Use lucid earlier 'before' going full gcd/shield heal mode, not once your mp is too low 40 -50%.

Also, as you aoe dps, target the tank while doing so and hit ogcd heals. Key here is that at least you can help kill things faster. Apply a buffed shield first, aoe, reapply before the buff(s) runs out. Rinse and repeat. Dead mobs can't hurt tanks. If all else fails, just focus on healing. Longer mitigations first and keep regens on cd dont save these (it's a dungeon).