The best tanks I know pull with a towershield and then hotswap in a buckler after a few seconds when the debuffs land (meaning your parry and defence mean somethign now narrowing the gap between a buckler/tower).
If you feel unsure about tanking in offensive just swap into it halfway into the fight but honestly block is the only avoid worth anything initially. Block and Health. Defence and parry (which you are prob near the caop of anyway) are very ineffetive vs an undebuffed epic mob so by the time they become useful the epic would be debuffed to such an extent that anyone could tank it.
So in short: Initially all you are vs epic is a big shield and a big bag of health. After the initial hit as debuffs start to land its all a about holding agro locked to you... and for that as a zerker you need dps and lots of it, offensive stance and buckler line best way.
Just promise me you'll pull the mob with a tower shield you'll make the healers feel much happier
