Slitherine

The main objective is easy enough on the hardest setting but I haven't been able to reach all the bunkers/ towns in time.
Any tips?

IIRC I wasn't able to pull this off myself on hardest difficulty. If it can be done you'll have to go offensive westward as soon as possible, but this very risky with the constant Japanese attacks. It's also questionable whether the reward will still worth the losses you will take from such aggressive playstyle.