Get mace magic (secondary) or axe (first).
Mana Bomb is only good with Triple Fatal + Loop, but consumes alot of TP (the 2rd ressources for use skills after Mana), Dark Trace or Pentagram do more damage with TF + Loop than Mana Bomb anyway and 0 TP = no Spirit Assimilation, or Boost.
Mana Bomb should be lv 0 (I removed mine) and the skill behind it should be lv 0 too (I don't remind the name).
the skill u absolutely need to ignore is Darkness Culling (the aoe on Mar skill tree), u can optionnally take the net bind (imo I don't have it, Rumir can dps alot and are only weak when silenced/debuff so getting paralyzed then debuff or full cc, or silenced and that skill won't help)
You go full psy don't go full vit like Equal2Nuts.