To counter Metallodas, use Metal Slash. One metal slasher probably won’t be enough, as there are so many of them.
Also, getting a Barricadus will help too as it will absorb a lot of damage from Metallodas. Don’t underestimate the damage from Metallodas, it deals ~2xx damage each hit and only needs 5-6 hits to take down your Ark.
Or if you would like to try your luck, you can try this, this is what I did:
Level 99 Don, Showdown
You need to cast Haste on Don, I used Raioh. After casting haste, he can cast Stun gift.
You probably need some Flash bombs before that to buy you some time.
Equip the item that can boost Wind Arks’ stats.
This can take down the last 21 Arks before Haste wears off. So your team needs to deal with the first 13 Arks, the more the better. Note that the Arks left for Don must be a multiplier of 3, because Showdown’s damage will go down significantly if there are fewer than 3 Arks.
The key - pray for some Bonus actions.