So, if you kill Temmie in the first encounter, you immediately reach LV 2, with increased stats and everything.
However, the way it works is that it instantly sets your EXP to 10, then gives you a LV increase, along with the appropriate stats.
Regardless of whether or not you should have reached that LV.
So if y'all can find a way to trigger the Temmie encounter multiple times, you can reach LV 20 before even encountering a single enemy.
Of course, the drawback is that if you reach a higher LV than intended, your HP and Max HP are set to a blank value (due to the fact that since you are never supposed to get higher than LV 20, there's no reason to assign stats to any LV above 20, which causes the game to attempt to set your stats to a non-existent value) and you instantly game over.
Of course, since you'll be at LV 20 with only 10 EXP, it will be very hard to reach the LV required to reach LV 21.
Why am I telling you this?
...
I don't know.
I just felt like it.

Welp, until next time, have a good one!

-KK