This is a good battle but I believe Mei ultimate wins(due to it being old hiruzen).
doton vs suiton
Hiruzen's most important element, during this match, is doton because it is superior to suiton. With this elemental advantage and the scale hiruzen is capable of bringing it to, hiruzen is able to comfortable defend himself from mei's suiton. Mei won't be able to take this battle too easily. In fact, she will be at a significant disadvantage
hidden mist
Mei's hidden mist will be completely ineffective during this match as hiruzen's able to track his opponents through a sense of smell. Using that, he can comfortably track what mei's doing in mist and, in fact, he could use the mist to his advantage. Once mei executes hidden mist, she would eventually realize that it's more a disadvantage for her than hiruzen and would ultimately release it.
CQC
Using this in an AoE is what can tip this battle into mei's favor. During the battle with orochimaru, we learnt that the kusanagi could hurt enma, in pole form. Now, kusanagi is just a sharp sword so, if a sword can injure enma, concrete dissolving acid should equally hurt him. Of course, it won't eat him away instantly, but it will sting and burn. While this jutsu is up, enma will be reluctant to enter that danger zone, especially for prolonged periods. Any quick strikes enma attempts could be easily dodged by mei - it's hiruzen's prolonged taijutsu which will be difficult.
The acid also serves the purpose of keeping hiruzen at bay and outside of CQC because, let's be honest, he'd pretty much smash her in CQC. Enma CQC is one of the deadliest things for mei in this battle.
Lastly, the acid also serves the purpose of keeping any shadow clones out. Any clones hiruzen sends in is guaranteed to die. Due to his limited chakra reserves, using a strategy like that is counter productive.
Assuming Hiruzen made it into CQC before mei set up her boil release, the fact that mei breathes out acid and spits lava will eventually push hiruzen back, if not to at least get a breather, at which point mei can set up her defences.
tl;dr; boil release: skill mist jutsu is perfect for keep hiruzen at mid-long range
mid/long range
due to the acid mist, this battle would now take place at a distance. That means it's mei's suiton/lava style vs hiruzens elemental jutsu. What wins it for mei is the fact that hiruzen can't through out too many jutsu. At these ranges, mei's at quick a disadvantage. Hiruzen's doton jutsu can block both her lava and suiton while he can use earth jutsu under her feet, raiton jutsu, water jutsu, etc to pummel her with attacks. Mei will mostly be on the defensive at this point but that's fine because she's also capable of blocking what hiruzen throws at her. Hiruzen's best bet will be to summon shadow clones and toss attacks at mei from multiple directions but if that attack misses it's over for him.
EDIT: If hiruzen plays his cards right and out smarts mei(really depends on mei's intelligence), he can pull off a win but his chakra reserves really crutches him in a match where mei can control at what distance it occurs.