Make it brutally hard and force the PCs to win it. The most memorable combats are always the very hardest ones. And if the PCs fail, it makes for an excellent backdrop for your next campaign.
Although now that I think about it, it'd be a very interesting premise for a mini-campaign to have a party have to deal with the coming of the End Of The World (As We Know It). The hordes of darkness approach: you have no chance. The women and children look to you to save them. Do you futilely throw yourself in the way of the horde and take down as many of them as you can? Do you lead the women and children away to find some sanctuary from which humanity will rise again? It'd be an interesting way for the players to FEEL the consequences of their failure, and to have a hand in making some of the legends that the next campaign will speak of.
Although now that I think about it, it'd be a very interesting premise for a mini-campaign to have a party have to deal with the coming of the End Of The World (As We Know It). The hordes of darkness approach: you have no chance. The women and children look to you to save them. Do you futilely throw yourself in the way of the horde and take down as many of them as you can? Do you lead the women and children away to find some sanctuary from which humanity will rise again? It'd be an interesting way for the players to FEEL the consequences of their failure, and to have a hand in making some of the legends that the next campaign will speak of.