I'm at level 10 in nightmare mode with a dodge tank Fenris (1:1 Str:Cun, Turn the Blade and Lyrium Ghost and a few +Def% accessories), Anders (Group Arcane shield and Heroic Aura) so Fenris has an 80% block rate against normals and Lts and 60 something against elites.
Enemies that that use activated talents/spells are your biggest threat, as they never miss. so it's best to CC or CCC (You can get the upgraded pummel strike and chain lightning at level 6) those enemies into submission first, if you see a talent/spell winding up Stonewall/Barrier/Running are imensely useful (I just thought, if you have a warrior Hawke, taking the Templar tree will allow you to stop any enemy from using talents/spells, might be awesome).
Eventually spells won't be a problem as I'll get Elemental Aegis, combined with Arcane Shield that's an 80% elemental resistance plus with Lyrium Spectre, Deflect and that one passive from the defender tree you can have a sweet 45% magic resistance.
If you re-spec' him, Fenris can have an 80% (or near enough) block rate against normals as soon as you get Turn the Blade and Lyrium Ghost (Level 7), it'll take a fair bit of micromanaging though avoiding spells, talents and any LTs, but with defense and awesome AoE attacks he'll tear through the rank and file while the rest of your party takes care of the trouble makers.