Keep trying - you'll get there eventually, and be on to much more difficult challenges before long. As far as AoSh goes, you're not gonna do well kiting the mastermind around corners with intuition like you might with other builds. Your ideal inventory as you approach the end of Hell should be a couple sets of strong armor, preferably modded, a reasonable handful of health packs, and several stacks of ammo, because your best bet with this challenge is to get right up in the Mastermind's grill and blast away. Knockback isn't an issue, but it moves erratically and you'll want to stick to it and stay at point blank range.
I (or many other more skilled players) can give you better and more specific advice if we can see your mortems - feel free to post them in the appropriate
forum.
