These spots are great spots given the formations that are there. These suggestions only apply if Random and Fixed Encounters are set to Original
Imperial Base - On the way to the Sealed Cave. Talk to the guards, fight Commandos. Commandos give 14 XP per level per commando. So each formation will give 56 XP per level.
Owzer’s Mansion - Fight the Nightshades (23XP per level per Nightshade), the Dahlings (31 XP per level per Dahling), the packs of Soul Dancers and Cats, and the Stillife. Those should bring you up a decent number of levels. Not likely to be enough on its own to grind you up to final strength.
Burning House -Balloons give just over 16XP per balloon per level. May be slightly better than the Imperial Base since you can get packs of 6.
Kefka at Narshe Battle - The soldiers marching up here are decent quick battles. They respawn once, so you can fight each trooper twice. If you have good AOE damage this is an always open spot that can be worth your time. The enemies are between 6-12 XP/level so they are not the best XP per fight, but since it is always open its not bad.
WoR Narshe outside beginners house - This really depends on what the random encounters there are. If they are beatable, and you can beat them quickly this spot may be good since there is a super fast heal right in the beginner’s house bucket.
WoB South Figaro Cave/Phantom Forest - See above these have easy quick heals.
Random battles wherever you are - When all else fails take the random encounters where you are.
Sealed Cave Ninja - The right-hand switch summons a single Ninja fight which is 25XP per level. It is only a single Ninja, and they can be mean enemies. But if the random battles there are bad it is an option.
Phantom Train Ghosts - Talk to ghosts and you will fight them. They are worth so little XP (4.8XP per level) that it almost certainly is not worth it. I guess if the train is full of evil? They do give good magic points.