"you want super difficult end game content well buckle up
Squad assault is seriously crazy. Map 3 had such ridiculous stat inflation I don't even know. Everyone had upper 50 to low 60 hp, attack stats in their 50s, high defensive stats...I'll probably let this sit on the back burner until we find out how cosnistent these maps are.
Just beat it. The five maps are the same no matter what, in the same order, and they only get skills if they had skills to begin with in the corresponding story mode chapters.
Thankfully, while everyone's attack stats inflate to gigantic proportions, their speed stats do not. If you can squad up with units with 35 speed, they can double nearly every heavy hitter in the run. Pack in some Hone Speeds, Spur Speeds, and Rally Speeds. Any enemy with higher than 35 Speed has ~45 attack to compensate with the exception of Navarre.
My team comp for the five maps were:
Map 1 - Two Catrias, Clair, Michalis
Map 2 - Masked Marth, Zephiel, Bride Caeda, Mist
Map 3 - Tharja, Wrys, Spring Chrom, Rody
Map 4 - Lucibun, Olivia, Tiki, Priscilla
Map 5 - Robin, Nino, Marth, Eliwood
Each healer had Rehab , Masked Marth had Axebreaker specifically for Camilla (her 33 Speed prevents Masked Marth from doubling with her 36), and Robin had QR and TA2 attached.
The hardest part of the run for me was the Archer in Map 3. You need to have a mage or archer sturdy enough to keep that Archer occupied and killed while your other units prepare for the two cavaliers coming in, or else that Archer will provide backup and pick one your guys off. Tharja worked here - just enough bulk to get into full Rehab heal range for Wrys to patch her up, and she doubled with Darting Blow afterward.
Squad Ace 1 Seal is the exact same as HP +3 btw.