My first guide video on AC! This EX stage is relatively easy I think. You can view my clear here

Recommended Units:

Holy Brawler: Preferably Shayna, however you can bring a Lucian or Yomi to solo the right side. This will take longer and is more dangerous since you need someone to take on the left side. This can be a strong water unit like Reagan or Lofia.

Sniper: Preferably Reagan, but Caris or Almira serve the same function. Acts as bait for Zahar and helps kill the knights and Dark Diablo.

Beast Tamer: Preferably Almira or Lucian so that they can solo the right side. Alkemono Mascot on Almira will help her take more hits.

Holy Knight/Cavalier I used Vettel, however Selena or even Lofia should work fine since the Fire Diablo should not do much damage to them.

Logi: Very useful against the Dark Diablo. He can easily do 1k with his Master Ability. After that, he'll just whale on the remaining knights. Alkemono Mascot ensures that he won't die from Dark Diablo.

Team slots should look like this! Bring potions too.

Zahar:

You want you first round to end up looking like this. HB should use Sharpening Focus and your sniper should attack Zahar.

After Zahar comes down, use Focused Palm to avoid his Counter and then attack with Logi.

Your HK/HC should go to this exact spot in order to get switched with the dark Diablo.

Dark Diablo:

This part is relatively easy. Just use Logi's MA and attack with him and Shayna until dead. Make sure your HP is high enough to survive it's counters/AoEs. Shayna and Logi should be blocking the Diablo so that it can't move and attack your sniper. I didn't do this and my sniper almost died.

Thunder Diablo:

When you start the fight, just make your Beast Tamer go all the way to the right. Do not stray away from the edges and go the maximum distance. The Dark Diablo will switch with units on particular spots so this is extremely important.

Just kill the first knight and move onto the thunder Diablo (2nd thunder knight may not aggro onto your HK/HC). If you have to normal attack, do it at an angle so it doesn't counter.

Fire Diablo:

Just send Shayna to the left after the dark Diablo is dead. She can kill it by herself very fast. Use potions if necessary.

Remaining Knights:

Your HK/HC should keep running back towards the starting point, leaning towards the fire Diablo. Once you've killed the fire Diablo, get Logi and your Sniper to help kill them. If necessary, bring HB Beast Tamer to help finish them.