...then I don't think defeating the Doctor as such is necessarily her plan. After all, she considers him her boyfriend now, right? And no, the Mistress being in love with the Doctor is not as far fetched as it seems - think the Joker/Batman with the Joker getting a gender swap. Then think back to the odd vibe between Simm's Master and Tennant's Doctor. There's a very thin line indeed between hate and love. And the Doctor did stop the drums beating at last...
Missy gave him Clara as a means to get him to Trenzalore to answer the Question "Doctor Who?" - her plan A to free Gallifrey. With that having failed, Missy was forced to buy time and grant the Doctor a new lease on life. The endgame is for Missy to work together with the Doctor to spring Gallifrey from the 'Heaven' only she (so far) seems able to travel to and fro from. To do that, Plan B (which we've seen throughout this season): She's got to drive the Doctor and Clara apart and to convince him that, at age 2300+ years (at least) and fourteen incarnations, it's time to start thinking about retiring from being Savior of the Universe/Guardian of the Pudding-Heads.
But despite the best efforts of sleeper agent Danny/Orson Pink, Clara's too much in love with adventure and exploring the Universe to give it up. And despite what I suspect was a spiked dose of regeneration energy twisting the new Doctor's personality a bit (forcing him to act more like a traditional off-putting Time Lord and compulsively work out with chalk block transfer computations that can break Gallifrey's stasis, among other things), his innate heroic Doctorness is now reasserting itself.
So, time for Plan C - the direct approach. And since Missy and the Daleks aren't really on speaking terms anymore, she'll have to call up the next best thing...