This mission is fairly straightforward, although things can go wrong if you dawdle or time things wrongly.
- Destroy all indicated targets in the allotted time.
- At least seven B-29 Superfortresses must survive.
- Destroy the train.
- Destroy 30 installations by the railway.
- Sink the Japanese convoy.
The mission begins with you in control of an F4U Corsair. Equipped with rockets, this part of the mission is straightfoward - simply destroy all designated targets within the six minute time limit. Make liberal use of your boost to quickly close the distance and eliminate all the designated targets.
The first set of targets are a pair of patrolling Gyoraiteis. These are easily enough dispatched with machine guns. You'll then get visual on the remainder of the targets - the Minekaze-class destroyers Minekaze and Hokaze, two more patrol boats, and several gun turrets. Everything except the destroyers can be sunk with machine gun fire; use your rockets to sink the Minekaze. It is already heavily damaged so a single volley of rockets is enough to sink it. Also of note are several cruisers docked at the bay; the Mogami-class Suzuya and the Kuma-classes Kiso and Tama. Ignore these ships and focus instead on the large retreating convoy to the south heading west. Sink all six ships in this convoy to complete the bonus objective. These ships are very slow and are vulnerable to machine guns, but feel free to use rockets for a quicker kill.
Keep in mind you have six minutes to accomplish this objective - it usually is more than enough time, but choosing to pursue the convoy may lose you quite a bit of time. Destroy all other targets but leave one standing so you can destroy the convoy. It is possible, but far more difficult, to sink the convoy later.
Once you've cleared the area, your control shifts to squad of P-38s escorting a wing of fifteen B-29 Superfortresses. These planes are heavily armed and armoured, but still require your support from A6M Zero attacks. Losing nine of these bombers will cause the mission to end in failure. Fortunately the fighter waves come in manageable chunks, and the B-29s contribute considerably with their many machine guns. The biggest danger is crashing into a friendly bomber, which may destroy both your plane and the bomber you're aiming to protect.
Try to engage the Zeros as far away as possible from the bombers, as they are focused on the bombers and not you. Once you get behind them, they will often fly in a nice straight line for you as they make a beeline for the bombers. Make particular note of J2M Raiden squadrons - these planes sport very powerful guns, and are designed to shoot down the heavily armoured bombers. Destroy as many as possible and keep your bombers protected; even B-29s can't survive long under Raiden attacks.
All in all, down approximately six or seven waves of fighters before a cutscene kicks in. The enemy is loading a train full of supplies. The bombers destroy the bridge but the train gets past them. Destroying the train is now up to you. However, you must also destroy thirty specified military installations in the valley. The train must cross virtually the entire valley in order for you to be able to accomplish this objective (unless you have a good memory of which targets to destroy; they can be destroyed pre-emptively).
The train proves to be a very difficult target - it is armed with a single rear-mounted triple machine gun and is described as "a tank on rails". To make things a lot easier, disable the machine gun by shooting it. Eventually it may respawn, so disable it again if necessary.
Make continuous strafing runs on the train, making sure to not crash into the ground. Also be sure to check your TacMap and your HUD and eliminate any of the grey circled targets for your secondary objective. Reduce the train to as little HP as possible without destroying it. This way you'll be able to destroy it in the event you screw up.
The train follows a winding path down the valley. Destroy the installations as they appear. The train ultimately closes in on a large tunnel. The mission fails if the train manages to enter this tunnel, but around the same time the train also passes by a large refinery complex. The refinery is your last-ditch effort to destroy the train. Simply aim for the large white fuel containers by the railway and destroy it as the train passes by. The resulting explosion destroys the train and earns you the Trainkiller achievement.