Project Wingman

After you've played through the campaign once you'll unlock all planes.

The last on the list, the Mk 1 PW, is incredible.

My strategy in most missions was just use the multi burst lock on missile to take out multiple clusters/flights at a time. Maybe rail guns for ships or the big airships, fly away during the cool down, and then use em again. Rinse repeat.

The gun is also now the heavy gun, with exploding shells.

For the final mission boss fight. Line up a railgun as you go for a head on pass, unload a cluster missile, then focus on one plane until its dead. I would just use standard missiles, and the gun until it was knocked out. Sometimes you can use the cluster, but the hit are inconsistent.

Once down to the final boss plane, he has 3 stages. Adding attacks at each. The first, just dogfight normally.

Stage 2 and 3 gets much harder to land hits. You can fly away till you get some distance, then about face and line up a rail gun shot/gun hits on your head on pass. Then just gain distance and repeat.

This difficulty mode is unlocked after beating the game once on any other difficulty. This is actually beneficial to you, because attempting this with any of the "regular" planes is unnecessarily difficult. Therefore, play through the game once on Easy or Normal and then you'll have the PW Mk1 available.

This plane makes everything significantly easier, for two reasons: it has physics-breaking speed and maneuverability and a special type of missile, the BML-U, which can lock on 10+ targets and absolutely swarm them with pretty accurate projectiles. Moreover, its guns are very powerful, to the point of destroying Phantoms for example in a single hit. The Railgun (RG) is also useful for Airships, but you'll mostly use the BML, the guns and the standard missiles. Still, some missions require some strategy on Mercenary, so here are my tips:

M6: Machine of the Mantle
There are a ton of ground targets, so equip the BML-U, focus on Priority targets with and let it rip. It's vital you keep your distance from that valley of lava, as it's packed with SAMs and Airships. Keep downing fighters that get in your way, as on Mercenary they're actually dangerous in this kind of situations. As soon as you're done with the helpless ground targets, focus on the Airships. A well placed Railgun shot can destroy them in one go, but using the BML never hurt anybody, as long as you have ammo. After that's done, just watch your speed when flying in those structures to destroy the Extractors, remembering that the PW Mk1 is way faster than your average fighter.

M8: Clear Skies
This is the one with the civilians, so be careful with the BML-U. I never used it if a civilian aircraft was in my FOV, so stick to standard missiles for the FC-8's, which are defenseless either way and the Railgun for the Airships. Eventually, you will face another squadron of fighters, where the BML-U will shine, especially if you turn around as soon as they spawn and hit them as soon as possible.

M9: Stepping Stone
This one's the first actually tricky mission on Mercenary. The way most targets are clustered together could influence you into using the Chimera and multiple MLAG hardpoints, but that plane is pretty slow compared to the PW Mk1 and you'll also face some fighter jets. Start by carefully taking down the ground targets, the BML-U will seem like overkill, but it won't matter. The key in this mission is to always destroy the targets in batches, then retreat to let your missiles rearm. After destroying a batch, move to the next one on the map, without never venturing too deep into their lines, as multiple L-SAMs can hit you from 360 degrees. Eventually you'll come up to a structure in a mountain, it 'll be to your one o'clock if you keep your initial heading. Watch out for some Tomcats that take off from there and the Airships that spawn to your 10 o'clock not too far from there. Don't get swarmed. Fall back, the PW Mk1 is untouchable on full throttle, then come around again and hit them. Repeat as necessary.

M11: Cold War
Some would simply throw themselves into the proverbial swarm of missiles this dogfight turns into as soon as you get in range of the enemies. Instead of staying in that ball of death and losing your mind with the constant blaring of the warning sound, as soon as the mission starts, switch to a Priority target, shoot the BML-U after having acquired as many targets as possible and then either do a U-turn or "joust" the enemies by getting on the throttle and ripping through them to turn around later. Regardless, this is the only way to thin their numbers enough, to the point it's actually viable to get inside, get behind one after the other and use the guns () to destroy them.

M12: Midnight Light
Needlessly painful, unless you know what to do and do it at the right time. Destroy the priority targets as instructed, I'd focus on the Railguns first, as they do the damage of a missile on Mercenary. Resistance from fast movers is negligible, a few Accipiters and irl fighters will spawn, but they're nothing you can't handle. The BML-U is the obvious choice for the targets on the oil rigs and the boats/ships. Eventually a flock of fighters will spawn, along with the boss, an SP-34R. It's imperative that you kill the fighters first, so you can focus on the boss. Follow the joust technique. As soon as they spawn and depending on where you are, turn around, punch it to gain a lot of separation, then turn around and use the BML-U. You need to be quick because the boss will still hit you here and there. After you're all alone, switch to the standard missiles and keep launching at it, while trying to inflict damage with the guns. The guns are the actual way to beat Frost, so don't think the missiles will do the work, about 70% of them won't even hit her.

M16: Wayback
Frost is back (kinda cringe), with another plane this time, but the same tactic applies. Use the BML-U and jousting maneuvers to eliminate her supporting cast. Pay a bit more attention to the railgun she fires, you only need to avoid it every so often. She will let you shoot her a bit with your guns and the occasional missile, then activate her AOA module and as soon as you've turned to follow her, you'll need to avoid crossing her railgun lines. It's nothing you can't handle.

M17: No respite
As you've probably understood by playing the missions I haven't mentioned, some of them become a cakewalk with the PW Mk1. The only think you need to watch out for on this one is to keep some of that BML-U ammo for those experimental... landships, which spawn at the final part of the mission. The Railgun can also take out multiple "nodes" if you shoot it at a shallow angle relative to the landship.

M18: Return
See M11: Cold War.

M19: Red Sea
Nothing to say here, since there's nothing you haven't faced before, just note that on the third part of the mission, there will be a plane carrier pretty deep in the naval formation. You can actually destroy it and prevent some jets from launching altogether.

M21: Kings
This is the epitomy of jousting. Firstly, in order to avoid a seizure, make sure you fly over the sea. Flying over the exploded earth will cause shakiness and ridiculous color saturation, making it even harder to see whatever part of the HUD is still functional.
Get on the throttle, put as much distance as possible between you and the other PW Mk1 and then turn around and hit the other fighters with the BML-U. Eventually they will all go down. Instead of following the PW Mk1 like the previous boss fights, keep jousting him with the BML-U, try to squeeze a double standard missile volley or railgun shot too if you can. After you run out of BML-U, switch to the standards and start the regular dogfighting, trying to get behind him and follow his trajectory.
After depleting his first health bar, he will start shooting railguns, like M16. It really isn't an issue. After you deplete his second health bar, he will also start shooting spheres of energy, which will damage you if you fly through them. Again, these will only delay your attacks, as you can easily avoid them with the PW and then find him again with your HUD. Just keep trying to damage him with the guns and the standard missiles in between all the dodging, it's a matter of patience.
After depleting his third and final health bar, the bottom right part of the hud will disappear, but I think it's impossible to lose at this stage. Regardless, keep doing the same thing until he dies.

TL,DR version:
- Beat the game on any other difficulty, then use the PW Mk1 for the Mercenary run.
- The BML-U wrecks everything on screen, very effective on ground targets and on squadrons of fighters as soon as they spawn in.
- The PW Mk1 is super fast and nimble, make sure you fall back using its superior speed to rearm your systems.
- Boss fights need you to destroy the minions first, then 1v1 the respective boss.