[quote=@Xaltwind]
[@BurningDaisies]
They were outright immune to spells up to tier 6. Our characters are all - at most - capable of casting tier 6 spells. The Medusa on the other hand, may be capable of higher tier magic. Ergo, Magic Resistance won't do much most likely.
[/quote]

I was thinking it'd give her, in DnD terms, a +2 or there abouts to her magic saving throws, or advantage on her saving throws. Nothing like immunity, she has specific cases of immunity listed, and petrification doesn't fall under any of those. (Although I'm sure she can, EVENTUALLY, regenerate petrification, but it'd take weeks.)