not really a guide, just a heads up to others, in case they run into the same issue:
I have finished my last 3 campaign mission today, received the final congratulation screen and the achievement stayed at 92% completion rate. I have even replayed the last training scenario, nothing. Then later today noticed, that the achievement have popped an hour later, when the game wasnt even on...
So dont get scared if the same happens.
Lets add some guide related parts as well, not that the game needs it... There are 8 training and 30 campaign scenarios, unlocking one after other with completing the previous ones. Some in the early 20's can be a bit more difficult, but dont get scared, the last 2-3 will be very easy with plenty of resources and a weak AI. Place your ships always to the 4 side rows / columns, use airplane carriers / torpedoes to recon, mines / carriers / single shots to destroy afterwards and you will win 90% of time. Placement to the sides is the key, cant remember losing a single battle since I started to do it.