Diarmuid Prisma Code event tips.

if in the worst case you dont have access to any powerful sabers you dont need to get him to 0 each fight because the progress carries over. if im not mistaken

I beat him (also other bosses) easily with my friend's lv100 all Maxed Jalter with MLB Kaleid Ruby

Offer his master a self Geass scroll...if you know what I mean ;)

I don't have any good sabers, so I just muscled through it with Kiyohime Lancer, Hans, and Waver. I just tried to fill up everyone's NP bars during the first wave, then unleashed them once I got to Diarmuid. I ignored his reindeer helpers and just went straight for him. Since I didn't have class advantage, I couldn't one-shot him with Kiyo's NP, but I could at least take off a 400k hp chunk right at the start. From there I could generally kill him before he released his NP (using the stun from the Chaldea combat uniform can buy you an extra turn once his NP bar is filled up, and Waver's NP can help buy you more time). I had a lot of bad luck on the very last fight (horrible card shuffles, Hans' NP defying probability and missing almost everything every time, boss using his evade at the most inopertune time, etc), so that one was pretty rough and came down to the wire, but all the others were pretty smooth and easy.

I would have preferred to borrow a nice Saber or Jeanne Alter with MLB Kaleid Ruby, but I don't have any decent supports other than Hans, so I opted to borrow a Waver instead.

I use double helena and attila, also works with a buster single target with mlb kaleid ruby,