Delete MANY MANY unneeded icloud emails

I have a friend who has SIXTY THOUSAND (!) unread emails in her iCloud email account. Most of them are from mailing lists that she at one point or the other subscribed to; in other words, nothing of substance that she needs to keep. What is your advice on the best way to delete/archive/remove/get rid of them, without having to click to select each one and then delete? I know how to sort by sender or date or subject, but still we'd have to click each one to select it.