I'd go for something along the lines of ransomware. The challenge is going to be the backups. Backup managers are paranoid individuals, and rightly so. You're going to have to zero out all of their backups without anyone noticing.
The first step would be to hack every single computer on the planet. I'm not going to say "how," especially for those which are airgapped, because an answer like that would have the NSA drooling. Let your AI figure it out.
Once they've done this, start encrypting financial data. As long as every single computer in the entire world has the exact same financial-data-encrypting package installed, and every single copy performs flawlessly, nobody can distinguish the difference between the encrypted data being pushed onto the disks and tapes and the unencrypted data. During this period the AI hopes that nobody takes an analog reading of any of the data and realizes the 1's and 0's aren't right.
Next step, is to do this for a few decades. You need to do this long enough to make sure any CD-R based backups deteriorate. This means keeping up with your hack of the entire world the whole time as new technologies come out. It might be easiest to leverage those hacks to put a few backdoors in the new chips so that its easier to keep them all infected.
Once that's done, simply issue an erase message to every computer with financial information, telling them to erase the keys. You'll have to figure out how to get all those computers (including those behind an airgap) to erase at once, but given that you've managed to hack every computer in the entire world for multiple decades running without detection, maybe you can pull it off.
Now all of the information is encrypted, but the key no longer exists. Sayonara financial data!
Or, you know, you can always go SkyNet. Take over all the nuclear weapons in the world, and obliterate everything resembling a backup tape storage site. Very few backup tapes are rated to withstand being in the locus of a nuclear explosion.