Sorry, no cheats as such for level 20s, but you could try fighting in an arena. Missions 16x, 20, 23 and 29x all have arenas. In mission 16x you should train up Erk, Canas, Bartre and maybe Matthew. Upgrade the class of the first 3 by mission 23 and use Erk, Canas and Bartre in the varena on that mission as well (also train Dart if you got him). These guys are super powerful if you train them well and you can complete the game with ease.
Just watch out for a tough enemy when in arenas, as a fight could turn nasty at any moment. With some luck you will have these super fighters strong in moments!

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