Garen Ult vs Maw of Malmortius

In an ARAM. I was playing vs a with ... She was the Villian. she was about half HP or so, (she had her shield from her skill up so slightly difficult to tell exactly). Anyways, I ulted her face. It literally did 0 damage. Her shield appeared on top of her current skill shield and original HP when I ulted her.

I thought the HP gets stopped at 30% when it goes off, and you get a shield. Not a that gives a shield when nuked

Plus, with villain on her, doesn't that give me some true damage? So I should have done some damage right?