Her full combo wont do very much damage against Mura especially if he’s in Avatar, but it’s hard to think of any mage who would.
Jaina with full Q build could keep spamming decent damage and slows at him though.
If mages with AA builds don’t count (like Falstad) and it must be a spell caster (or a spell build), the only hero that could reliably keep chucking nukes at him (and deny Second Wind) healing is Chromie, but that would require extraordinary aim.
Another one would be Ming, but getting constant W cooldown resets against a diving Muradin could prove difficult. Spamming calamity or close range missiles at him wouldn’t do much either.
Orphea is a great candidate as long as the player can keep up constant Q hits without a single miss.
But this is all just theorycrafting, as Muradin can counter a lot of mages and nobody generally drafts mages to counter tanks.