If he does get his ice shield up before you kill him you can bring it down by setting him on fire. This can be done with the Oil plus Fire damage combo creating an ignite. Alternatively, you can use the Burning Strikes shard to do the same. A word of caution, if he is ignited before his shield goes up then you will have to wait until the ignite wears off before igniting him again to bring down the shield. He is a threat while this happens, so stuns can hold him still while you wait.