Classic MMO style, I love it...have it be a phase, he triggers it when you get close and try to dps, he procs a shield, which goes away soon but leaves a damage field around the boss for a few seconds, if you play it right you can still get close range dps, but it has to be sporadic and guerilla style. Another similar tactic is to make a phase where the shield repels more damage the closer you are to it. long range and rockets/grenades/high caliber rounds still make it through at any range with reduced damage at closer ranges, but small arms like shotguns will be harder to use during this phase without range attachments.