Cannot remove shower diverter to replace

[Chadwick003]

I have a shower diverter that started leaking. When I attempted to remove the shower diverter, I continue to strip it even more and cannot break it loose. Loose. I've tried a sock it, channel locks, vise grip, heat, penetrating oil, and nothing helps. Break it loose. This is a Tempress II. Any tricks to break this loose?

 

[Breplum]

No tricks. It is just screwed in, obviously.
Deep socket with at least 3/8" drive is the best tool. Metric ONLY.
Heat: If you have the replacement, and cartridge above is pulled and nothing plastic left except the diverter, controlled heat might help.