Marni weakness lies on foes which can do double hit brave weapon or skills that can allow that such as Desperation and most fatal against unit that can bypass her Miracle weapon effect Heidrun weapon effect.

I just saw fortifications on her, & that works pretty well, too, for a B skill on a tank build. Ofc, i could never get more than 1 brave alfonse.

>>2 TBF, winter Edelgard can disable the miracle aspect with her weapon, on 2nd hit, or if someone else triggers divine vein flame on Marni (preferably the latter, since Edelgard may fie on 1st attack, as she's blue vs green. So many have flared sparrow, etc, that's not hard these days).

The tank build needs something to increase special count, IMO. Maybe a breath ring instead? I didn't see the marth ring on that build, as mentioned in the comments, but that's an option, too, for some people.

>>2 We initially have her in 9.0 but are currently testing her out to potentially promote her to 9.5. Our main reason for 9.0 is her lack of an accelerated Special trigger on her weapon, forcing her to equip Emblem Marth's ring and only have Escutcheon for the Shield Fighter build. She could've had Pavise instead if she had an accelerated trigger.

>>1 Furthermore, her weapon also has a built in miracle effect which outclasses B! Seliph's, meaning she can live on forever as long as the enemy doesn't land a second hit after the effect triggers or neutralizes the effect. This means she's a great counter to some of the dominant metagame monsters like B! Felix (without rare NFU support), W! Edelgard, and E! Ike given they don't have fatal smoke.

I feel like Marni should definitely be in 9.5. When invested in properly, she has potential to arguably be one of the best near save tanks in this meta. Merging her to +10 (easily done thanks to her accessibility) will grant her an insanely high stat line, which her preferred weapon takes full advantage of, granting her bonus stats and TRUE DR.