Yes, I know, it doesn't star the Master Chief but it's still set during Halo 2's event. Our own review editor Mr Slater thought it was "a gorgeous, surprisingly deep shooter that sucks you back into Halo's universe."
Surprisingly, it came out on iOS with Halo: Spartan Assault, another game that first came on Windows Phone but it's not yet discounted.
Still, we heavily recommend Halo: Spartan Strike, especially if you're a fan of this franchise. Go kill some Covenants!