Retribution ius not worth investing points in, I had it all the way to 10 with my Lone Wolf and it's not that great.
If you want a real good tank and have a 4 man party, consider putting leadership on your tank and giving the rest of the party slightly more constitution than your tank.
The tank will not get the leadership bonuses but you can just give him the best armor (armour is much more important than constitution), this way your party will eventually come close to being immune to elemental damage and as long as the AI targets lower constitution they will target your tank. (he also has to be closer to the enemies melees to get targeted)
