The knock-back is backwards and seems to be a fixed distance. Whether they are together or spread out you should be able to run directly forward to get back into whatever range you need. Also, if you have the SR aura I imagine it should do a lot less damage. In any case, if you run out when he banishes you will significantly lengthen the duration of the encounter.

Also, after the knockback but before the portals you have quite a bit of time to get into whatever position you need. He will go back to the prior red beam tank until the new red beam appears.