If you're way overleveled for this, and are doing planar flags / MB for a specific reason, then just sit in his room and kill the spiders as they enter. The spiders have to walk inside a bit, and go to the 4 batteries that are on either side of the doorway as they enter. That's where they explode to power MB.
Since the explosion is what powers the machine, you just need to prevent them from doing the explosion. As a result, MB will become targetable in 5 minutes. This essentially overpowers the script since the few seconds to swing, retarget, swing, run to the other side of the room, and do the same 2 more times is easily enough time to get MB targetable.
I was able to one shot all the spiders with a 115 Bard, so even with the weaker melee classes this should be possible. A move speed buff longer than 5 minutes helps since your going from one side of the room to the other as they come in, but I'm not sure if it's necessary with Run AAs. I have Selo's on all the time, so I figured I'd mention it.