On a guild raid the whole guild fights against a certain number of enemies that lurk on multiple levels of the dungeon. Finally, the guild tries to defeat a dungeon boss. The crucial point is that a raid must be done in one go, which means that the members' constitution is not refilled after one enemy has been defeated.
Now you might think "What the...." but of course your guild will be rewarded with something new and epic for having sent the dungeon boss to eternal sleep.... "