Congrats on the Achilles!
The ultra rare event dragons don't fight with ultra rare strength. They fall into the weaker Unbreedable tier. So it's no big loss, battle-wise, in letting them go (or leaving them in storage).
Common>Rare>Unbreedable>Super Rare>Ultra Rare
"Unbreedable:
Any dragon that cannot be bred is classified in this tier regardless of its game-displayed rarity. This includes dragons won from tournaments, dragons received by completing Dragon Album goals, Gold type dragons, and any other dragon that cannot normally be acquired through breeding. Some (but not all) expired limited dragons also fall into this tier."
A side note:
The Neo Diamond is Unbreedable, so it won't fight like the actually-an-ultra-rare Diamond Dragon.