Let's assume there are 45 creatures on board. You seperate them into three piles. (a 15 cards)
Pile A is chosen if the dice rolls 1-5, pile B is chosen if dice rolls 6-10, pile C is chosen if dice rolls 11-15. If you roll 16-20, roll again. (You could also take 6-size steps, so A is 1-6, B is 7-12, C is 13-18, 19 and 20 is a new roll)
After you've chosen the pile, seperate it again in 3 piles a 5 cards and repeat.
You will now have 5 potential survivor creatures. Assign creature A 1-4 of the dice roll, creature B 5-8..., creature E 17-20.
The key is to find a working divisor for the piles.
Though, don't ask me what to do if the number of creatures equals a prime nmumber greater than 19.^^