for abjurer merc: +skills, -enemy ligthning res, Bonus to energy till EF Bonus on Spell damage caps (30 ish %) , ligthning spell damage, fcr , and increased buff duration to make the superbeast morph last longer.
All those stats boost the damage output. If your char has sd boosting buffs keep them active on the merc.
Besides that you have to make him tanky. (all res, high base defense (TCD comes from Warp armor skill from merc), %DR if possible.
A lot of testing and actual playing is necessary to find out the optimum for yourself. Every player has its own preferences. But the above points are a rough outline for 'working' abjurer.