Do your DPSers just target whomever your tank is targeting for DPS? If so, just have your tank target the ice blocked person -- its not important that the tank isn't in range or isn't swinging away at the boss for a few seconds. I just would target the ice block'd person with my tank (usually the tank) and the DPSers would automatically start DPSIng on it after a few seconds (remember target switching has some lag).