So I've decided to go through the following classes with Nowi:
Manakete -> Dark Knight -> Manakete
I'm currently at Dark Knight, and the exp required to level each time is huge. I think I know the reason why...but I'm still not sure...
Anyway, if I switch back to Manakete after Dark Knight, will the exp still be really hard to grind off? I'm currently grinding in the EXPonential Growth DLC and it stopped helping, once Nowi accessed the Dark Knight.
If you have it, complete the Bloodlines 3 dlc asap. You are given a Paragon skill scroll each time you beat that map. Paragon doubles all exp gains.
Grinding Golden Gaffe is a fine way to grind as it yields decent growth while also fattening you coffers.
The ULTIMATE way to grind, once you are strong enough, is to pair a single unit up with Chrom in Bloodlines 3 (the map that gives the Paragon scroll). This map has 50 enemy units and zero allies to steal exp.
At capped internal levels you will gain a minimum of 8 exp per kill. 8 exp x 50 = 400 = 4 guaranteed levels.
Paragon takes +4 guaranteed level ups and makes it 8 guaranteed level ups.
EDIT: Since you are only at Ch 15, I'd recommend continuing with the story.
Also, abuse Spot Pass maps and solo them with a single pair of units for exp and renown grinding.