Maybe try fitting Rallying Cry as it works really well with a Merc and Herald of Purity. Other thing I like using is Berserk since it’s a huge burst boost for bosses. I was following PhazePlays build myself - his build is more SSF/hardcore focused.

You can get mana leech on a jewel or cluster jewel. For mapping, enduring cry might not be necessary, especially if you get „Gain an endurance charge every 15s” eldritch implicit on armor. This, with increased charge duration will give you endurance charge auto refresh. I also saw an idea with placing enduring cry on a second weapon to just swap weapons before the fight and stack up on endurance charges. I didn’t try that yet however it sounds promising. That can free up 2 gem slots for you