Huh, so I guess you can brute force things with overleveling after all. Just make sure to recruit bunch of low level units. Works for me, I don't intend to abuse it but it's nice to have some kind of plan B if I'm really stuck. If it would be possible neat way around it would be to grant some exp to all your units, certainly not as much as for those who participated in a battle but still you can assume they are doing something, I don't know like training (I mean if I remember right you do get some AP this way). That way even units you never ever use would get some levels and it would be massive quality of life improvement for people who would like to have big roster of specialists they rotate in and out.