Let's be honest. Kraken shell is off the charts op right now. It's strong enough just by the damage block it provides. It's like twice as good as vanguard just with the damage block. The kicker is it allows stun negation. You can run right through stun after stun just with the one skill. It's like a linkens and two vanguards combined.
I suggest making the level 3 kraken the new level 4. Lower each level slightly to make it more balanced.