Trenzar repop is at least < 2h (if like me, you messed and killed him too fast...).
The key is to NOT kill Trenzar too fast AND then, not kill Senvial too fast either.
If you're doing your Epic 1.5 late (lvl > 90 ?) : big fire power, beware of your damages - strip all attack buffs and ALL damage shields.
Load your lvl 19 ignite DD.
Engage Trenzar, get him slowly to below 30% or near, he will pop several elemental along the way, but more importantly, pop Senvial.
If you kill Trenzar too fast, he won't spawn Senvial.
Once Senvial poped, don't face him (prevent ripost), kill Trenzar and adds, just keep Senvial up. Get him slowly to around 7% with low lvl DD (ignite = ~600DD -> lowers Senvial by 1%).
Then, and only then , he will lose aggro. Hail him and Senvial will give you his Blessing + pop a chest with lvl 70 loot.
Hope this helps (I would have enjoyed having those info prior to my first 3 missed encounters...)