I'm starting a new playthrough and last time I didn't use a tank at all, so what is a good tank setup (attributes/abilities) for Aveline to be a tank class.

I would also like to know!

What I did was distribute my points evenly across Willpower, Strength and Constitution and then focus more on defensive and threat generating abilities. Once everyone crowds around the tank, just let your mages do their area of affect spells. I did this during my first run with a mage and it's proven to be super useful especially with the Ice/Fireball spells.

Her sheild will take care of defenses, adding strength will make her attack more powerful and keep her from getting knocked back as much, which is key. The main thing is making sure have the bodygaurd ability activated and targeted for your weakest character (a mage, probably). Otherwise, max out battle synergy (really, get everything in battle master). Maybe go ahead and max out Bravery as well. The key to a good tank is having the right sustained modes active.

Her top tier gear is oriented towards a str and con build, which wasn't my choice on my first playthrough (I gave her a ton of cunning for def), but Im trying that now.

Constitution (For more health)
Willpower (For more stamina, which you absolutely need if you going with much Sustained abilities)
Strength (Not to much here, because you will get immune to knockback - flanking skills and items later on. But enough to wear good weapon and shield.)

For skills i recommend you skill taunt and the upgrade for that immediately. The key for a tank is control. I would also recommend immovable from the guardian talents. It will reduce her attack speed and movement speed, but get's 2% damage resistance each hit she takes. As for others it really depends. I always skill up the passive abilites for more armor, damage resistance etc from shield and weapon, defender and guardian. For elemental bosses elemental aegis is not bad. Shield defense is also good of course. You could go for a defense vs armor type of build, but that's going to be hard to balance, especially on hard or nightmare difficulty.

As for tactics: Taunt, taunt and more taunt. Allies takes damage = taunt. Enemies clustered = taunt. ranks elite or higher = taunt. Boss = taunts.

Now, this is my setup and it works really well for me, so i can't say if it will work for you and your playstyle.

-Cheers.

Having a rogue with Goad helps too.
 
It is like an extra taunt.