Most of people have no clue of their class potential, they copy/paste the build and composition, do big numbers and they expect the same build to be effective everywhere, it doesn't (always) work like that. *edit : and burn boss, ignoring mobs.
What's the issue ? > Too much adds, too much projectiles, condition/daze.
What is the solution ? > Adapt. Overcome.
FB, tempest, scourge, ren/ventari ... Aoe, reflect, stab, some condicleanse or resistance ... A fb with Virtues lane, F2 F3, Wall and renewed focus can just cancel 80% of the nuisance.