Ok, so here's what you do.
1) use a build that can shut down opposing actions
2) when the boss battle comes up, state with an air of challenging smugness that you'll buy dinner for whoever gets the killing blow on your BBEG
3) shut down everyone's PCs except for Sam (just don't be too obvious about it)
4) when she finally kills you, humbly admit your defeat, then look her in the eye, smile, and say "so... pick you up at seven?"
contingency
5) if she balks or if someone else got the killing blow, save face and offer to take everyone out for pizza
6) buy Sam an ice cream/cheesecake/dessert instead
since you won the encouter, who is sam and where are the pics?
Yeah, figured someone would ask that sooner or later
It would be hilarious if it were Randalthor