To clear Ace Combat 8 Mission 15, equip air-to-air weapons, close in through the enemy ECM, and destroy both large bombers by attacking their MAIN ENGINE targets. Wait for their climb to gain on them as their airspeed drops. Stop their dives toward the allied fleet, and keep firing missiles if their defenses block your attacks. Both bombers must go down for MISSION ACCOMPLISHED.
What should you equip?

Recommended starting point: a fast aircraft suited to air-to-air combat, with air-to-air weapons. Speed helps you pursue the bombers; no specific aircraft is required by the available clear instructions.
HPAA with damage upgrades: coordinated kills
This setup supports a plan to finish both bombers almost together and avoid the harder second phase. Maxing HPAA damage helps, but is described as optional for the approach.
The reported upgraded configuration takes 4 HPAA per MAIN ENGINE. Treat that as a configuration-specific result when planning the following sequence:
Stay with the fleet initially
Destroy one main engine on each bomber
Prepare both remaining engines
Finish both targets almost together
The simultaneous-kill plan is a player-tested approach, not a guarantee for every configuration. A fixed HPAA hit count across all difficulties is not confirmed yet.
F-22 with QAAM and 4AAM: sustained missile attacks
An alternative setup uses an F-22 with QAAM, 4AAM, and relevant speed and air-to-air missile upgrades. Cycle through all three missile types, firing repeated salvos into the engines while pursuing the bombers; the reported setup has the first type reloaded after the three-type salvo.
An Ace-difficulty clear with this setup achieved S rank in less than 10 minutes. That is an individual result; the formal S-rank time and score thresholds are not confirmed yet.
How do you get close enough to damage the bombers?

- Close the distance to the enemy formation. Strong ECM severely reduces your lock-on range, so attacks require a close approach.
- Once close enough, select a MAIN ENGINE subtarget and concentrate your attacks there.

Destroying an engine is progress, but the mission requires both bombers to be destroyed. Continue attacking their main engines and watch for their dives toward the ships.
When can you catch them, and how do you survive the later defenses?

Wait for the bombers to climb: their airspeed drops, creating a pursuit opportunity. When they dive toward the fleet, interrupt their attacks on the ships.
During the later engagement, a rear optical weapon makes following directly behind dangerous. Avoid that position and stay outside the blast radius of self-destruct UAVs.

Keep targeting the remaining bomber's engines at close range. If the defensive system stops your missiles, maintain missile fire so some attacks can get through.
What confirms the mission is cleared?

The clear condition is both bombers down, followed by MISSION ACCOMPLISHED. A destroyed engine or a single bomber kill is not the finish.