Thief shrine is the best place to farm xp and get your charactrers level up faster since you can challenge the dungeon as many as you want. Defeated enemies respawn once you go out and re-enter. You can use other locations that enemies respawn as well for sure.
How to win battles fast
1. Change settings
Once you start a battle, go to the settings menu, and make "Game Speed" fast and turn off "AI moves".
2. End your turn
You don't have to move your characters but just skip your tuen. Enemies get close to your characters, attack them, and get defeated instantly.