AP wise Marvy shouldn't have that big of an advantage, as she scales below 27 megatons while Sayaka stomped someone who is 6 megatons by existing
Skill wise Sayaka is highly outclassed and, coupled with Marvy's enhanced senses, getting an hit in is not going to be easy for her.
But Marvy doesn't have anything to incap, so her only way of winning is by destroying the soul gem. While Sayaka has the range advantage, it's not going to matter much as they start close so the fight is going to be in CQC. Sayaka should resist the status that Marvy can apply
although they work on tier 2s, so they shouldn't be a factor, while Marvy doesn't resist the sleep manip.
About the soul gem thing, it's totally in Marvy's ballpark to think about trying to destroy it, as in her first appearance she appeared out of nowhere and hit the weak spot of a mech (iirc it was some sort of crystal on its head) which was weakening the CPUs. Also, some of her normal attacks are often imbued with elemental properties with effects like these:
TL;DR with getting some good hits in Sayaka has a good chance of winning, as if she inflicts sleep it's over. But that is going to be quite difficult with Marvy's superior skill + enhanced senses that let her see Sayaka's blind spots, coupled with her versatility in CQC. Overall, I'd say Marvy Mid to High diff, mostly due to Sayaka's regen making her hard to put down