Next tactic to slow them down, mess them up. Asinjin is going to create 3600 square feet of dolorous motes in front of the gate, leading up to it. This is when the first group gets close.
there are 36 motes, so everyone is going to have to make a bunch of saves. I understand that the anti magic is going to shield them, but they are not all covered, and fewer now that some groups are spread out. Every time one enters, it will fail its save and stop. It will snap out of it when a anti magic hits, but that won't be enough time for it to move and get out before it is re-effected, at least in some cases. The running fields pass over in less than a round.
It should create a log jam that slows them notably. Also, it will eventually weed some out in the motes after all the fields pass.