I find the best thing to do is follow the manouvere I designed that I call...
Mack The Knife.
Named after a great song though that has nothing to do with its content.
See, this guy who is trying to poach the girl, he's out to get you, stop you, take you out (of the equation - not to dinner) so he gets the girl. He is the enemy, the threat, the proverbial knife.
Mack him.
Not as in kiss him!
But I mean charm him, bond with him (not in the oh you are superior way), make him like you, or at least make him have to PRETEND to like you.
If he doesn't he will look like the assh*le he so patently is in front of the girl and she won't like him.
If he does pretend to like you, your stock value goes up in front of the girl because you can get the attention from girls and guys and she'll likely want some of it back.
And even if neither of you get the girl, as long as he's not the patent assh*le you may have found yourself a potential wingman.
And if he gets her (hey win some lose some, we can handle anything even losing!) then at least he's likely to put a good word in for you even if by accident by saying how great you were but somehow he pipped you to the post.
REMEMBER: The true warrior rarely needs to use his weapons, he evades and confuses the enemy, into weaker positions, or into his own alliances.