Conviction does not affect Poison resistances. You may get better results from a Defiance Mercenary (if you melee with your dagger), or a Holy Freezer (yet another element at your arsenal). Insight is a must if you will spam PNova/PExplosion.

Now, as for Stats, they should look something like this, I would recommend:

Str: something in the area of 100 base for an Archon Plate Bramble, 77 for Dusk Shrouds

Dex: either Max Block Rate, or none at all; have you considered using Poison Dagger w/ a dagger? If so, Wizardspike should cover your resistances and caster rate quite nicely (75 Dex), or even a Silence weapon (on a 6Os Crystal Sword, low Dex, low Str; +2 skills and +75 resistance). I don't think any of us have a Death's Web wand on us, now runewords, I can handle.

Vita: all of it, 2 per point, 1.5 per level

Energy: if with Insight Mercenary, none at all, 2 per point/level

You Mercenary should have the normal Mercenary set-up for Casters (ethereal if possible):
Holy Freeze Aura from Defensive Nightmare, a must
Dream Helm (Mercenaries do not share Facet bonuses, unless they have their own)
Fortitude armor (Boneweaves seem the easiest)
Insight Polearm (either on a low Elite or good Exceptional)

If you are considering using Poison Dagger:
Defiance or Blessed Aim from Defense/Combat in Normal (not Hell), or AI mercenary with Faith
Doom Polearm, or BotD Polearm
Fortitude armor (it's a Mercenary staple, to say the least)
Anything with +Str or Lifesteal should work: Stealskull Casque, Natalya's Helm, Guiellame's Helm (Orphan Set), Dream

Now if you'll excuse me from my rant, I have food to eat!