I'm currently stuck at the proper Mayfall battle. That Def-10 general is already miles more troublesome than Grunthall's suicidal AIs, and the game's equivalent of invincible Luca hasn't even made an appearance yet.
Like I said earlier, past Grunthall the ideal battle formation is Hero and one other infantry unit stacked with +DEF units and Regen units at the cost of Attack. Like this:
An example. The 'o's are where your mages should be, the 'x's should be hero or other infantry, moved just within one tile of an enemy unit's range to lure them over - because their allies will move with it - and have your mages swarm. Don't kill it unless you have a mage that can reach nothing else, you want to knock them all down to 100 men per unit to make them 'safe', and if you can get the enemy leader within range, focus all mages on knocking him down in the HP. Use your infantry with Regen on it to pick off weak units that can't counter, you'll regenerate 5% of your max units back with no penalty. As a sidenote, you know you can select any enemy unit to get a red radius of how far they can attack on any turn to make things easier, yes?
That should get you through it. A-Ranking requires effort. x3