While Mirri is tapped, there's no restriction on how many creatures can attack planeswalkers you control or battles you protect
Im off to find a horror movie that I havent actually seen yet
All you need is to survive one more of your opponents attacks so that you can untap and cast your game-winning spell
P/T Number 11 Language English Legal Formats Commander, Legacy, Vintage Not Legal Formats Alchemy, Brawl, Competitive Brawl, Historic, Modern, Pauper, Pioneer, Standard, Timeless Rulings (10/4/2004) You may tap any untapped permanents you control with the appropriate creature type (including this creature) to pay the cost of the ability, including creatures that haven't been under your control continuously since the beginning of your most recent turn.
Its too slow for anything but Commander, but its excellent there as a source of many continuous effects: cast from exile, card draw, life loss, and so on