Merric for Rein/Lyn counter?

It would be good to tank them, but I don't know that he'll have the raw power to defeat them. He's got the lowest atk of any mage. It could be doable with Luna. Just watch out for that Brave Bow and Cancel Affinity.

It does work.
I have a merric like this. Honestly he isn't the best unit ever, but he sort of work to counter them.
If you wanna be able to kill them when they attack I suggest you use Bonfire and QR. This way he will take 2 hits from rein and Lyn and counter attack twice allowing him to proc bonfire and kill them.