No credit is given to the Staff Master achievement for damage done by 4 of the 5 staff skills, as a Necromancer.
The only way I can get the achievement to update is by killing something with direct damage from the staff. Spamming Necrotic Grasp (skill 1) for credit is the only sure option. If any of the condition damages from the marks (skills 2-5) kill the mob, Staff Master does not update.
~RD