. The 110 will run back out, reset, re clear and tell the 90 to log back in before he starts on the final boss again. Rice and repeat and you should only use 1 potion and about an hour or so if you don't get locked out on a quick speed running class
Go inside the instance and wait by the entrance. Have the 110 or 99 clear the dungeon and before they kill the last boss use the potion. You'll get credit for the bonus (full level to about half a level 95-99) per run. Right away run outside and log out.