Round table

No, I get the idea, it's just that the description doesn't mention that you need at least 3 Centaurs to protect the Elves from the first attack.

You can simply divide the Elves in half and give half of them away. I don't think it makes a big difference whether 5 or 10 Grand Archers shoot at the Azure Dragon: D. Then, for speed, you can split the Elves into 6 stacks and place a Centaur next to them.