Some low cost items are very effectiv for their price (I'm thinking: Scaled, Scalemail, Banded). Instead of having 3 or 4 empty slot I suggest filling up all slots, then replacing one item at a time.
The assasin oak has alot of diversity and counter measuring, and buffs against the minions. The problem of full focus minion build is that you won't get any of these for a very long time. I believe it's worth it to sacrifices some damage for more survivability (shield), maybe soulpower I. I'd cut down on Moral/Pestilence to get these. Also I suggest getting the +10 dmg favor item. It's like 2 levels of Moral.
I honestly haven't tried this, but I think a hybrid having more survivability and only minor dps losses on the minions side could work. What you also want to do with a DPS-monster is constantly push buildings, therefore forcing the enemy to come to you, which in return disburdens the other team members.