Thanks for the comment!
First of all, you don't actually need to level up all the skills to beat the game. I think it's easiest to focus more on "finding things I can do/beat at the moment", and the EXP will come (the toughest enemies give a lot). Your biggest source of EXP right now should be the hard dungeons - if you find the enemies difficult, look for better equipment/allies/consumables (Bottled Explosions solve a lot of problems if you can afford them!).
If you haven't already done all of the side quests (found by checking rumours), they're a good source of rewards as well. Another one is joining the Academy which will open up some new areas (you can also discover those randomly by exploring or by gathering information with Charisma 2).
There's three islands infested with bandits/beasts/undead, respecitvely. You can grind for EXP there. If you defeat too many enemies, they withdraw for some time, but return later. This is quick if you're strong enough, or have lots of money and can afford bribing the bandits.
Exploring the islands and travelling has a chance for encounters with EXP, but I wouldn't call it a "quick" way of earning it.
False Mirlanda will appear at the Academy once you've achieved a certain fame threshold. But her staff is actually more useful for characters without the Magic skill, I would say!
Hope this helps! The game was designed for multiple plays, with lots of procedural generation, but the core structure remains the same and it gets easier/faster on subsequent playthroughs.
EDIT: Oh, I forgot about the gills potions! You can buy them from the Witch once you've brought her the ingredients - she'll tell you what she needs.