I completed this mission on like, my 5-7th try, I see no issue.
Thats aboslutely horrible, what the hell is this?
Issues in the numbers beyond my comprehension but in fact the key to this mission: Radar honestly, because without it in the balanced tech youre always gonna be outspotted and outgunned before you even see them - effectively making whatever you built useless.
Annoying, but you actually wanna go with Main Guns and fire control, and choose a slightly lighter caliber, 15" will do enough. Get good towers - accuracy is really important.
Once youre in, get in, start sniping, focus the bb, approach cautiously, use DD's to divert attention of CAs, and youll be gucci. You *can* also go at him head on, but like, why? youll only get your ass killed.
The biggest luck element in this scenario is positioning the enemy BB's bulkheads. Positioning means if you are able to instafire on him or not from the start, its good if you are, because then youll also probably be out of his spotting range, and knowing the AI, hell be woefully inaccurate.
Also speed. You need like a lot. My design is 32.5, which should be around enough, the enemy BB will be unusually speedy too.
Keep calm, and follow this design. Engine efficiency is 100%, mass and stability optimised to the max. Youll be solo, so radio is useless here (and youre really mass strapped)