I usually go till he eats one then I measure if I think at least 3 of the 4 left will survive the next 5 rounds till the next eat. If they will keep it going doing what damage you can... The important part is when he is ready to eat try to escape with your important remaining characters leaving the least important behind for him to eat... Since the cool down is the same over your next attempt you want to do this so you have 5 full turns of your fresh team before one of them gets eaten. So leave 1 guy behind in your previous attempt right before he eats to give the cool down a full 5 turns before he eats again. Also if he dies and you only have like 1 turn before he eats on your next attempt include an expendable character with a taunt to force the rancor to eat him saving the 4 other good characters for your damage