You first have to fill up your extend drive (the red EX bar at the bottom) and activate it once it's filled (default on keyboard is C).
Once you're in the extended drive, you can use your ultimate against him (default on keyboard is A).
When you've hit him with your ultimate attack, you will be able to deal damage to him until he gets his blue barrier back up again.
Since your ultimate immediately ends your EX drive, I recommend only activating your EX drive when he has the blue barrier up. When he does bring it up, pop your ult and hit him like usual.
He can't be finished off with any normal attacks either, so you will have to use your ultimate to kill him.
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