Going back to GD, need some pointers

Two new classes definitely look great, but I’m interested particularly in Necromancer for now. As he seems to be pretty good summoner, which classes synergize best with him in your opinion and why?

Also I have two chars: Witchblade and Spellbreaker. I’m quite unhappy with the latter, and I literally took Nightblade as a filler. Now Necromancer merged with Arcanist looks almost perfect to me and I want to try it out, but thought of slugging through full content (I left game in a middle of Ultimate) is kinda mood breaker. It would be great if there’s some way to reroll second Mastery by in-game means. But hope is thin, so other than that, I presume it is still possible to do with GD Defiler for example? Any precautions I should know about ahead considering new version and all?