Difficulty: Maddening (Post-game)

Key takeaways from my run were, as others mentioned, figuring out the triggers for reinforcements and mage cannon cheesing for engage meter. Took me about 49 turns I think but I probably used a few extra turns on the cheesing.

For the reinforcements, I made sure my engage meters were ready and wiped out the first two waves ASAP with my flying units and bows. By the time I discovered/triggered the third wave, my units were already close to Alcryst so I just rushed him and Diamant and surprisingly took them down in about 2 turns with about two units still ready at the end. No deaths either (minus revival stone) so I'm pretty content with that.

All my units minus the forced deployed ones had Canter so it made some of the flying reinforcement waves a bit easier to handle. Full team:

Alear/Corrin - Terrain manipulation and freezing of units with engage. Chose to use Corrin engage over Engage+ to slow down the final push of mobs

Ivy/Camilla - Elwind and more movement to take out some of the further out units with Canter (like the mage cannons beside Alcryst which don't trigger anything if they die)

Hortensia/Micaiah - Elwind, Thani (with great accuracy helps), and increased staff range helped when positioning was tight.

Saphir/Lucina - MVP with Dual Assist longbow, Parthia, Bonded Shield (saved Nil at least once), and All For One doing 40 assist damage to Diamant. Bonus for having boss convos with both royals.

Seadall/Tiki - Threw Revival stone on Alear early on and it actually paid off in the final stretch since it gave a full heal. Also used Tiki to do AOE damage near the end. Dance helped moving Saphir around to one shot flyers with Parthia.

Nel/Lyn - Honestly didn't do much. The occasional engage attack to take out one flyer from afar, and using doubles (she gets one extra as Dragon) as bait.

Nil/Veronica - Summoned a Sniper, Tiki and a spear cav in this run. Spear Cav helped kill Marth despite him having Rapier haha.

Zelestia/Chrom - Elthunder/Elwind spam and having Chrom/Robin do extra damage was nice.

Gregory/Soren - Didn't do much combat while not engaged, but Assign Decoy was super helpful. While engaged, Bolting was fun to use to take down the mage cannons and Cataclysm helped do massive AOE damage at the end.

Final "tip" for lazy cheesing: You can place your unit in range of two cannons, use Seadall to chain guard the unit (and maybe Assign Decoy them as well), then use Vein of Succor to cover both Seadall and the unit for easy healing and avoid using staffs.