Yes, Act II Town Guards own Rogues, Iron Wolves, and Barbarians 99.99999% of the time for a long list of reasons.
The best would probably be either a Holy Freeze (slows everything down for you but also shatters if you summon on the side), Might (can kill stuff really easily by himself), Prayer (with Insight means quick regen for you and massive spams).
Since poison is not covered by Conviction, you are open to a wide variety of choices. If you wish to upset the balance between your opponent's health and your health, use Prayer. Your opponent will get poisoned while you gain health. Might will increase the damage of your dagger. Also, a Rogue with the runeword Venom (Tal Dol Mal) on a bow would be interesting complement to a poisonmancer. Since you already do massive damage with poison I suggest a mercenary that will be able to either mow the minions down with light but quick damage (a Desert Mercenary with a fast weapon like an elite scythe would do, a Rogue, or a lightning Iron Wolve for quick hits but more or less guaranteed hits) or equipping a mercenary with poison based equipment (which leaves you to many options, but two examples would be putting Grief on Iron Wolves or Barbarians). You might also want to consider Holy Freeze, Holy Fire, or Holy Shock based equipment for the finishing blow when your opponent is at low health (~1 hp).
Yeah, Id be torn between two different types of mercs
1. Act II Desert Mercenary Nightmare Defensive - Holy Freeze
This guy will help you get your finishing blow, and given decent equipment (ie. obediance and fort with guillaimes) he will also deal out very nice damage.
2. Act 5 Barb Merc
This guy would be nice just for the insane damage he can deal. Give him a LW CV, fort, and guillaimes, and watch him and his new might aura tear the place up. You could also throw a dragon on him for the 1pt killer. This guy would be great with a fire golem as well, as he could deal dmg and holyfire from golem would be used to deal last hp.