How do you get the Tower key for Unova link to unlock black city on Pokemon white 2?

You don't need the Tower Key to unlock Black City in Pokémon White 2. You actually need the Link Key, which is obtained by completing the Unova Link storyline.

Here's how to get the Link Key and unlock Black City:

1. Complete the Unova Link storyline: This involves talking to the Unova Link representative in Floccesy Ranch and completing various tasks in different regions.

2. Travel to Floccesy Ranch: After completing the Unova Link storyline, the Unova Link representative in Floccesy Ranch will give you the Link Key.

3. Use the Link Key at the entrance of Black City: Use the Link Key on the lock at the entrance of Black City, which is located south of Floccesy Ranch.

Note: You must complete the Unova Link storyline before you can enter Black City. This includes collecting the Link Keys from the other regions. You won't be able to access Black City simply by completing the tasks for the Unova region, so make sure you've completed the storyline for all regions.