Master Seals and Master Crowns
When a unit is at level 20, you can use a Master Seal (if they are going to level up to second class) or Master Crown (if they are going to level up to third class) to have them change classes instead of using Experience. Since they are at level 20, you won't be missing out on any stat increases anyway.
More tricks, tips and cheats for this game are right here - Fire Emblem: Radiant Dawn cheats
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