Well the undead and demon bonus would be great in demonis and undead lands when your getting closer to level cap. You could go Rina and if she fails to give you a xp baby then you could go frog princess -although she only make a 5% baby
. Then go scout mirabella's babies and if they're better you could stick with her.
You could stick a bag over Gerda head and try for an xp baby from her too.
At least one of them should give you a xp baby though.
Finally, If you think your level is high enough you could swap to the demon wife and equip weapons so you can kill stuff better.
More xp, less battles that need fighting, faster time. (well hopefully) Could turn out to be worse because your army isn't as good as it should be.
Might go 3 upgrades of learning first, then to 2 on undead xp skill, then the last undead xp gain before I run into lots of undead.