I use this setup now instead of Multishot to farm low end GRs (70-80) in about 3-4 mins and T13 in about 2-3 mins.
It uses Lightning rune instead of COLD, because Cold pierces and kills things WAY TOO FAST lol. To refill hatred with Karlei's Point you need to double hit same mob with Impale. If mob is dead on first hit, you obviously can't hit him second time..
For lowest rifts this might be a problem with very high dps, so i usually use Blood Vengenace passive to completely solve the issue (just like for my no-generator Multishot build), or Seethe instead of Dark Heart.
Even easier option here is to just raise the level of the rift (right now i mostly farm GR70).
what legendary gems do you use? I'm still looking waiting for the travelers pledge to drop with a slot. I have a good ancient one. The only legendary gem I use right now is PAIN ENHANCER. It's incredible! I have it in the compass rose ring.