I am around level 8 or 9 while doing this.
-Buy a few skill potions in Dusk
-Fly to E4
-Go 3-W, S, 4-W, S, 5-W, 6-S
-Have your Cleric (Or at least the one that's the fastest) use the skill potion 4 times
-Open the Crypt (you'll enter a battle with a Lich and a ton of mummies in which the Lich may eradicate a random party member upon first turn, but whether it goes before your party, or even lands the hit. is random, if eradicated restore your save)
-Have your Cleric cast 1-8 (Turn Undead) it will kill all the enemies instantly.

You will get around 5000-12000 exp per battle (at least that's how it varied for me)
Also good loot may spawn afterwards, I got Ultimate Plate from one of the battles.

Here is a question.

With Skill Potions, is it feasible to do this at level 1?

(Of course, this means getting the Skill Potions at level 1 in the first place.)

Also, is there a way to use Fly at level 1? Maybe some fountain somewhere that makes this possible?

Sadly, not as far as I know.

But you can make a level 1 character (or a few) and have them join your higher level party.

SPOILER WARNING
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I THINK there is a fountain in the area north of Sorpigal that grants all spell levels to your spellcasters, but the effect will only last until the end of the next combat. Also, it's nearly impossible for a level one party to get to the fountain without being killed.
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END SPOILERS

WARNING!

In that same swamp, there is a rare chance you could run into a Gold Dragon, or multiple.

That shouldn't be too much of a problem.

If you run into Gold Dragons on the way to the fountain, you can just reset if you can't handle them.

Once you use the fountain, you should just be able to Fly (or Town Portal) back to town. Now you just need to reach the inn to save the game (assuming it works like MM2).

Also, I notice that Holy Word exists, so if you aren't high enough in level to hit the liches with Turn Undead, you should be able to use Holy Word with the help of the fountain.

One other question: If you wait until you get level 2 magic (probably level 3), could you just Jump over the squares that give you encounters, or does that strategy not work here?

There are no set encounters in the swamp itself but there are on the shore, though my directions avoid all of them.

Also, even further, I found it even more useful to only take my Cleric and one Sorcerer. As they level up so much faster this way. In turn, I can only imagine with both my Sorcerers and Cleric up a few levels they might almost carry the rest of my party against the tougher areas.