i am currently building the fire monk EP build with a wep and shield, trying to get the furnace but no luck yet, what is a good alt to the furnace until i can get it?
It does not increase damage by 15% but damage to elites by 15%. Therefore depending on your current elite damage it will add between 0 and 15%, definitely less than the straight out 30% of bane of the trapped (plus the secondary adds some crowd control).
The additional 20% does not last long enough to finish the boss in high GRs.
You could use a torch or 2 weapons with some cdr for T6.
Level 25 Bane of the Trapped, caused enemies to become "slowed", of which plays into the damage boost. At level 50, you gain 30% more damage to targets that that Gem has slown down. Bane of the Powerful is decent, but doesn't last long enough for Rift Guardian Fights.