It’s no rocket science really.
Learn which attack/phase in a fight deals a lot of damage and use tools available to you to reduce damage taken from you or the party.

Typically, you’ll use rampart/Sheltron/Sentinel for big single target tank buster.
Hallowed ground to cheese mechanics or negate damage when healers are distracted focusing something else.
Then convalescence/bulkwark in between to ease the damage a bit.

Use divine veil/reprisal for party wide damage.

Since all fights are scripted, the ultimate goals would be to map every CDs throughythe fight for maximum utilization.
Also realize that mitigation should be a group effort and that every class has tools to contribute.