Had this on hand for a while, but I don’t anticipate making any further changes, so:
This patch adds stat bonuses to recruitable enemies, and Roshea, based approximately on the bonuses generic enemies receive in Maniac mode; these bonuses were manually entered into character data and so will not scale down or up with the selected difficulty.
Naturally, some enemy-joiners who were already good (eg, Darros, Etzel and Sirius) become unreasonably strong under these conditions. If that bothers you, or you want non-enemy joiners to be improved as well, I’ve set up an alternate patch (link) which buffs a wider array of units. You can find the changelog for the alternate patch here.
Both patches are to be applied to a translated FE12 ROM file.