That's often not possible for any boss that moves around after you portal, or in open spaces. Plus in the time it takes to portal a few times and get the position right the boss is regenerating health, also if there are any mobs you can't control your army to focus a boss or remain stacked together, especially without teleport. Also if you corner a boss they can't be surrounded.
I think removing some skeletons and adding more archers would help with this problem. Or perhaps the melee attack range can be increased for golems or skeletons.