Twice I fought him and my ult did nothing. He did have maws but with 15--20% hp it should deal some damage right? I forget how the second one went but the first attempt he took zero damage. I recorded it but the match was long and rather not dig it up.
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Downvoted for asking a serious question?
Stay classy, Boards.
Anywho, answer is no. Panth's Innate only blocks basic attacks, tower pews, and OnNextHit type things. Garen's Ult is none of those.
Pantheon bought a 
at 8 minutes into the game, which will stop the magic damage Garen's ultimate does. Since he wasn't your lane opponent, I wouldn't worry about countering it and just build normally and use your ultimate on someone else.
I would recommend rushing Brtualizer into Sunfire Cape when your opponent decides to rush Hexdrinker against you as Garen. The Sunfire Cape aura will proc the Hexdrinker shield so it won't be available to block your ultimate and you have AD and Armor Pen to pushing their lack of armor.
