Don`t worry to much. There is a reason why she`s not with him and is with you and just be relaxed and cool like nothing special is going on. Also, if worried about him being agressive, take care of your body language. Try to look secure, dominant, relaxed. Don`t be edgy, scare or insecure. Stand tall, take your space, don`t get angry if he tries to pick a fight. For any case, it would be good that some of your friends is also on that party. Its potentailly problematical if you are there alone, without backup and he`s with few friends. That fact can boost his confidence and level of agression.
If thats the case and you know that he has bad temper, maybe you should think about changing your plans for another party (and your girlfriend too, of course).
But if you go to a party, and it escalates to violence, if you don`t have any chance to stop it on time, the best way to stop the bully is you own explosion of agression, scare the motherfucker
Never hit with wide haymakers cause if you are not in training, you will rapidly loose your stamina. Hit your opponent with fast jab/cross combo and aim for nose or chin. Then after calming down the first attack, take your girlfriend and go away, fast. Don`t run, but get away fast. After he/them recuperate after first shock, they might attack again and you won`t have help od surprise moment.
Here, there is the worst scenario but I think everything will go just fine and without agression.
Take care buddy and write a report how it went the day after!
Marco