sorry this is a Japaneses sub

If you right click on it you will see it has a hero on it. It can be taken out with persistent destroyer attacks. In my experience you will need at least three (3) to do it with the best sonar and depth charges available.
You will need plenty of anti submarine units for the second half of that scenario, so prepare ahead of time.

thanks mate

Happy to help

Note that attacking it BEFORE it has a chance to attack you for the first time makes things much easier. The sub has an ability called "deep dive" that greatly boosts its defence once it makes an attack (+6 or something crazy like that, though this is from memory). I *think* this ability lasts into the player's turn as well, though feel free to correct me if I'm wrong.

Damaging it a lot, killing its engines with depth charges, or even outright killing it are all much easier on the first turn / before it attacks.

truthfully that unit is just op... even against other subs

its gilbert islands mission

Use torpedo bombers, for whatever reason, they do much more damage than your ships