You need to go back to title after step 3
Only doing auto train does not save the game so you can abuse that

e.g your character is at 20 and you do auto training for 100 turns and they lvl up to 25
the level up of 21 and 22 sucked and 23 was decent so you take a exam for two times
profit

the steps i had figured out are
1. start any chapter and save in front the inn (the more turns the better)
2. do auto training that gets you exp for one character at the inn for all the turns - 1
3. keep a memo of the level ups
4. do exams for the number of that sucked

average fe experience
but if you are interested taking a exam(doesn't matter if they pass) moves the seed for level up
e.g if you are lvl20 and take a exam the next level up will be what was supposed to happen at 21->22

apperenty a way was found

it is busted but the exp gain is much less than normally punching the units

that pary was rally sex