The DNS Exploit is a method for accessing old Mystery Gift events in Gens IV and V. In Black and White, one of the items you can receive is the Liberty Pass, which will allow you to catch Victini. This method requires configuring some settings on your DS, but as of 2023, it still works. It could be argued that this is hacking, but all you're doing is reaccessing old, legitimate events -- the Pokemon you obtain will always pass hack checks.
It can be a complicated setup process, so I would recommend watching this video or reading this question, which provide step-by-step instructions for how to use the DNS Exploit.
Here's a list of possible gifts from DNS.
