I believe the game's polearms consist of Caranbrot, Jassiran Glaive, Pike, Poleaxe, Storm Glaive, and the Straight Pike. Of these...
- Caranbrot is good for Strength/Toughness builds, but your evasion will suck.
- Jassiran Glaive is good if you wanna use Charm attacks in addition to just hitting people.
- Storm Glaive gives you a bit of spellpower for a spellsword (spellspear?) build.
- Straight Pike gives you a bit of evasion if you want to build around that, like with Duelist's Stance.
The gameplan is fairly straightforward: Get non-heavy armor, your polearm of choice, and whack shit. For armor you'll want something like Lamellar or Panoply of the Holy City. They have decent defensive stats, but because they're not heavy armor you still get the extra attack power from Veteran. If you're running just pure strength, Equanimity is always solid as a stance. Charge Weapon if you're running a spellsword of some kind.
As for "off tank", that depends on what you mean exactly. If you mean somebody who takes aggro when the actual tank dies, that's easy. Do big numbers, maybe have a threat generating skill like Dominance or Bull Rush ready just in case. If you mean someone who actually swaps aggro with the main tank, you're gonna need to micromanage your party by manually dropping tank stance on the main tank, and then having the off tank switch stance on and use a taunt. It's.... remarkably similar to tank-swapping in an MMO, actually, now that I think about it.