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I actually got it after midnight last night.
It wasn't easy.
To get it, you have to obtain every. last. medal. In all four and a half acts:
Some of these things were simple, some of them were delayed because Act 1's first "fix" just got rid of the quick fix.
A * denotes something I believe only counts as meeting the conditions on a replay.
Act I:
Ice Breaker: At least 100 ice fragments [2 runs]
Last Hitter: Finish off the mama Spider as Evan when you fight it a second time, then choose to kill its babies*
Proper Hero: Kill a total of 400 Demon soldiers as Evan in Act 1. [3/4 runs]Note: it took me 4 runs through to get enough kills (although I don't know how many I killed the first time around [I believe I may have switched to Aran for the DPS]) (I didn't count the total number, so the kills may not count the first time around)
Act I.5
Freud's Leadership:
Aran's Strength
Lumnous's Focus
Mercedes' Speed
Phantom's Freedom
Shade's Compassion
I used this guide: https://www.reddit.com/r/Maplestory/comments/4t2usd/heroes_of_maple_act_15_medal_guide/
Act II:
Shattering Pot: Destroy 70 Jars in the Ruins of Abraxas [2 runs]
Ignorant of the Truth: answer the question with anything other than "shadow" twice, then answer the question with "shadow"*
Out of My Way: Kill the Gatekeeper 3 times or more [3 runs] (literally all you have to do is replay the act twice)
Act III:
Master of Disguise:
Clear the Perfect disguise stuff without making any mistakes (select "Dr. Lim", first set: [numbered left to right, ordered top to bottom] #2, #4, #1, select "ask for it&quot (Second set: #3, #2, #2, select "Steal it from him"
Mission Possible: Clear the sneaking portion with Luminous without shutting off the circuit breakers to turn off the Lazors, or skipping the lazor section altogether. (the third circuit breaker is obligatory, therefore it doesn't count.)
This really ought to be called Mission IMPossible, even at the risk of a lawsuit, because its freaking hard.
I don't know how I did it, forgot to record it, so all the advice I can give, is to be patient and observant.

Final Act:
Damien's Adversary: Defeat Damien. That's it.
Herbicider: Defeat 30 flowers. Another quest, similarily to Shattering Pot and Ice Breaker, a few notable differences: Flowers are annoying obstacles, not background elements, they have hp instead of taking a set number of attacks, and they respawn almost immediately, which makes this one possible, and even easy to take on in one go.

Unfortunately, if you were hoping that this could all be finished in one playthrough of the Final Act, you'll be disappointed by:
World Tree Conqueror: Use Every Hero to defeat Damien at least once: Now, this isn't as lame as it sounds. You will NOT have to replay the Final Act 6 times to do this. You only need to do it 3 times. However, you must take EVERY HERO WITH Evan AT LEAST ONCE! This takes 3 runs because Evan HAS to be the first member, meaning you can only take 2 of the other 5 at a time
Here's how I did it: First run: Evan, Aran, Shade. Second Run: Evan, Mercedes, Phantom. Third Run: Evan, Luminous, Aran.