Corner the boss in Dal Gurak's Tower and slay him quickly. Second before the cutscene, use spacebar to select and enter to pick up, as quickly as possibly, his dropped shield. You need empty room in inventory cleared beforehand. You might also accidentally pick up his sword, which is quite useless (everyone swings it clumsily). But shield is 5000 endurance, it never goes down even with heaviest of hits and it almost always give you complete immunity against all attacks, when blocking; provided that you are facing towards the hit and you aren't too close to it.