Sweet and simple, pray to your gods if you meet a Mary while petrified. A solid 3x hit with a status, and a very very nice bonus if she procs it, there is no real flaw to Mary's skill.
Mary should be played as a typical 3-hit regardless of her boosted damage state, her attack will let her deal damage just fine. Theres no need to wait for a petrify or attempt to get the 2x bonus, it will happen mid-volley with all likelihood and is honestly simply a bonus to her skill. Being the only Red Strike attacker (bar Charlotte) that does real damage, she will always be in your team during strike weeks.