"...Strategic waiting?"
Sorry, mate. As many others have said, your girlfriend's right. And (promise I'm not saying this because of female solidarity, but...) she sounds like she knows what she's doing. As you've said you're "pretty sure she'll beat you one on one," have you ever thought that she's just giving you tips? I give tips to my gamer friends all the time when we're playing. (a la: "No, when I do that move you want to block and counter, not try to attack me after I've started winding up for it" in a fighting game.) You can fight about it if you so desire, but to me, it seems like she's in the right.
Worst comes to worst, and you need to duke it out in a non-fight situation: pick a game you're both equally good at and decide who's right in a 1v1 match (or best 2 out of 3, or whatever you can agree on). Winner of the match is right. Though, really, talking it out with a simple "I'd appreciate it if you don't backseat game while I'm playing" would be simplest.