The easiest way is instead of focusing on your sub characters, on your previous turn before using MB, find four of your characters that already have 140 morals and make sure they're next to each other. Next turn, separate your current twin unit and attach all the 140 moral characters in twin. Then use Maximum Break.
If you're evenly killing of your grunts. Then most of your main characters should have 140 morals.
The best Way I've learned is make 2 twins right off the bat out of the ones you want to get the Max break with. Then once the leader of each twin reaches 130 Morale(Shouldn't take too long I have 4+ twins every mission going between 142 and 150 and i just reached stage 18.) Then Push Square before selecting that group to change leaders and then get a few kills with them. That will usually get both sets to 140.
Edit:Forgot to finish what I was saying lmao.
Xbox 360 GamerTag: HolyKnight22 PSN: CrimsonDivide
Zengar, being relatively high armor and possessing little to no ranged weapons, is really easy to get morale with. Throw him into a group of grunts, they'll attack him because he can't counterattack, guard the attacks, and you get instant high morale. This kinda works with Kyosuke - both characters you need lots of morale for. This helps alot when there's not enough grunts to go around.
Not a big fan of Maximum Break though. Pretty much the only way to take out most retreating bosses, and because of it Kyosuke ends up hogging all the boss PP drops.
Kill all the grunts, making sure everyone gets a few kills. Go into the menu settings and turn on the will display, that shows you the will of every unit on the map so you don't have to press a button all the time to look it up.