Courage of the Colossus
I'm pretty sure someone made a post about this already but if you are getting 5 man hard CC'd the shield is useless because they're not hitting you. Also I think most of the time when that Malphite dives into your team and you see the enemy running behind him you're not thinking "OSHT WE GOTTA KILL THIS MALPHITE RIGHT AWAY" and start putting all your damages on him. I don't think the mastery is that OP, it's great, maybe will be a bit tedious to deal with, but not super over powered.
Disclaimer: Of course I haven't even seen it yet so I'm not sure but I imagine those huge 5 man stuns won't be that great for COTC
