Originally Posted by flink

In case of emergency, you can reload/close the page before the end of the fight. You will lose the used raid point but you will do 0 damage. Also you will not get any reward unless you damage it again (preferably with a more appropriate team...).

This trick also works with any map run as well; you'll be able to retain your stamina and try again for a better attempt, although you can't select the same ally you chose before until cooldown wears off. It also only works twice an hour; anymore and you'll just kill your stamina.

I think this trick was found around the time the stamina bug became a thing.