Observations
Okay, so I've made many observations on how things went. According to Mike, EXP growth is fair up into the Level 20s. From then on, I've seen complaints of EXP gains being slow from people starting with the lower spectrum of the current target levels (late 40s). This is, bearing in mind, that the next equipment sets, when they come, will likely expect players to be in the Level 50s or even the Level 60s for warriors. That and I would predict Moonchyllus would need to be fought by players who are in the early Level 60s. Lastly, as it currently stands, the Blue Shell, currently the highest-leveled item in the game, is unreachable to all but the Top 6 (soon to be 7).
These are the observations I've taken from both the game, what's currently in the game, and what's up next. For the levels themselves, I had to make a scatter plot for the EXP needed to get to the next level.


X (left) is the player's current level, while Y is the EXP required to get to the next level. As it stands right now, there is expected to be a spike that may render story progression past Moonchyllus very tedious. Bear in mind, Eliatopia's got a long way to go.
The Experiment
I've used Desmos to experiment with the math, mainly because the best way to visualize the data is with a graph. Here's what the data looks like:

X is the current level, and Y is the amount of EXP required to get to the next level.
Red is Jakub's EXP formula
Blue is the scatter plot for the current XP formula
Green is my EXP formula after using Desmos to experiment with it. Took me a few hours to come up with this guy.
I had to use regression (the act of estimating the line graph needed to match a scatter plot) to figure out the best approximate formula for what we currently have. This was the results (good luck reading THAT!):
0.028911653391x^4-1.0829858877792x³+31.31330020046x²-360.9326573697x+1065.153196796
The reason why I did that was because I needed to use summation to figure out approximately how the EXP formulas will stack up against one another. I'll start with Level 1 through Level 50 for accuracy reasons. Here's what I got when summing up the
Blue Formula (current): 1075195 (approximated from regression equation)
Red Formula (Jakub): 2670671
Green Formula (Flashlight237): 770752
Using the estimates given, here's what I got.:
2670671/1075195=2.4839
770752/1075195=0.7168
For the road to Level 50, you will need 2.5x more EXP to get there with Jakub's formula. That is not going to satisfy anyone at all. As for my formula, you would need about 0.717x as much EXP to get to Level 50.
Now, how about from Level 1 through Level 60, which is the target level for the battle against Moonchyllus?
Blue Formula (current): 2773595 (approximated from regression equation); 1698400 more XP
Red Formula (Jakub): 4752381; 2081710 more XP
Green Formula (Flashlight237): 1456309; 611180 more XP
Now let's see how this holds up.:
4752381/2773595=1.7134
1456309/2773595=.5251
Jakub's formula is still slower, but it's 1.7x slower this time. My formula, on the other hand, shows that you need 0.477x as much EXP to get to level 60.
Now for the last level: the Blue Shell (which should be equivalent to equipments from the first shop that will appear after Moonchyllus). Yep: Level 76. We're going from Level 1 through 76
Blue Formula (current): 9573418 (approximated from regression equation); 4821037 more XP
Red Formula (Jakub): 10050835; 5298453 more XP
Green Formula (Flashlight237): 3323621; 1628403 more XP
Let's see how this hold up.:
10050835/9573418=1.0499
3323621/9573418=0.3472
Jakub's formula is 1.05x slower than the current one, so its break-even point must be just a tiny bit beyond Level 76. With my formula, however, you would only need 0.3472x as much XP to get all the way from Level 1 to 76. This should hopefully be fast enough to allow the story to go fast both up until Moonchyllus and beyond Moonchyllus.
Of course it may slow things down a smidge for the road to Lumbercore (which is a boss meant for those in the Level 32 to 35 range), as shown here:

But it's a little too troublesome to flatten the curve in that area, so I guess it's a small price to pay.
The Results
After hours of graphing a formula that would work best for players, I have come to the conclusion that the EXP formula that would best suit everyone is as follows...
XP to Next Level=3x^2.5-12x+30; where X is the player's current level
This formula should be fast enough to allow story progression without having to worry about the crazy spikes that are expected to happen with the current "EXP to Next Level" formula when players go into and beyond Moonchyllus. Yep.
What About Monsters?
Monsters are easier to handle; at least in my opinion. The general rule of thumb seems to be that a monster's power is twice that of its defense. Because of this, I'll just make a tiny modification right... here.
Monster EXP=((HP/5)+Defense*2+Power+Poison Damage+Fire Power)/3
That way, stone wall monsters like Leaflings and Stalagmites can play catch-up with powerhouse monsters such as Grunts and Night Stalkers. Granted it might skew things a little in the powerhouse monsters' favors as well, but still, the goal here was to make the stone walls more viable.
Conclusion
With my analysis done, hopefully more meaning is put into perhaps the single most demanded modification: "fix the XP formula."