<div style='font: 9pt ; text-align: left; '>Whenever your characters are promoted, they are treated by the game as level 21. You'll notice that a just-promoted character will cost 210 gold to revive.
This includes treating the character as level 21 in the experience tables.
So you can promote a level 20 character. They get exp like they're 21. They have the stats of a level 21 character (which is slightly lowered because of the stat hit you get for promoting).
You can promote a level 30 character. They get exp like they're 21. They have the stats of a level 31 character (again, slightly lowered because of the promotion - but it's roughly the same stat hit the level 20 character took).
You can promote a level 40 character. They'll get exp like they're 21. They'll have the stats of a level 41 character.
Get the drift? You can promote anytime from 20 to 40. You could promote a level 20, a level 25, a level 35, and a level 40 all at the same time, and they'll all get the same experience from enemies there on out.
Therefore, you want to raise your characters as high as you can stand to before you promote them. They'll be much stronger, and they'll level afterwards just as quickly.
The best place to do it is the secret battle in the elf town. Killing mist demons there will get you to level 30 fairly easily. You could get to probably 35 or so on them before they tapped out, but I was getting really bored at 30).
You don't need to. The first time through the game I promoted everyone at level 20, and I had a nice challenge for the rest of the game (not super hard, but not a cakewalk). To be honest, if it's your first time through, I'd promote everyone at 20.
Going higher will make your party more powerful, but speaking from experience, if you promote everyone at 30, you will basically rampage over most monsters the rest of the game, and it does take some of the fun out.
Up to you. 25 is a nice compromise if you don't want to promote instantly but don't want to wait forever to do it.</div>
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