Seriously? Mei neg to low diffs him...
Old Hiruzen is inferior to Mei in every single thing.
Filling up an area with her Futton takes splits of a second. Moreover, her Futton was shown to be useable at
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as well.
Coming close to Mei isn't even an option, when she can easily use her Futton Spit or Lava Globes, which she can fire off like 7-8 in ~0.5 sec.
Moreover, you're massively overrating OLD Hiruzen.
His stamina is terrible and chakra reserves laughable. Even without clones all he could do was to throw some irrelevant Katon and Doton. If Futton somehow doesn't end this, Mei's massive Water Dragons are overkill. Especially since she's shown her control over them and their movement.
Even though Doton has elemental advantage, it won't help Hiruzen if it's times weaker than Mei's Suiton. Mei traps Hiruzen in her Water Dragon like she did to Madara and either pushes him into the ground killing him with pressure or pushes him into the barrier which sets everyone who touches it on some magical fire
There's literally NO way of Hiruzen winning this. He cannot burn her since she's resistant.
He cannot crush her or break her bones since she's resistant to it somehow as well. Using clones is a death sentence for OLD Hiruzen and due to chakra reserves his jutsu are inferior.
@Edit.
Sorry for lack of scans, but I'm on some old pc right now and it takes ages to even browse the manga. But if something is unclear, I'll take my time