How to Save a Shape Changer From Radiant Damage

I cast moon beam on them in a cylinder that will catch all of them in the cylinder. They need to roll a constitution saving throw. If it fails, it also instantly reverts to its original form and can't assume a different form until it leaves the spells light. And so they all three get hit by 11 damage and they are all three blasted into puddles of black. Yes. And phenomenal form, but we've got to work on our like our formations.