I never used Jahan, I Lonewolf and Glasscannon my Tank and he gets heavily buffed by the mage ( Wildfire, oath of desecration and Fortify are my faves). I try to keep the enemy so supressed, busy and worried about my tank that the mage and Archer can just work the crowd. I also recomend 2h weapons for the tank I think they have a longer reach ( especially when dust deviling or Whirlwinded) but I may be incorrect.
Also this may seem basic and obvious but, concentration of attacks is really important, taking enemies out of the fight ( killing them or supressing them) is the only way to decrease the enemies offensive strength, because enemies fight at 100% capability until killed. So everybody concentrate on each target in turn until dead or supressed, it is okay to attack targets other than the one that is the primary threat if the toon cannot attack the primary target anyway or target is suppressed for a couple of turns.
If it isnt hard enough for you, try playing on hard and playing a smaller party. And there is in the works from Larian a new content patch that will have a hardcore mode, but ther is no ETA on that per Raze.
