Wooh! This is exciting! Glad to see that some community impact might get into the game! :D Oh and hi Battle Elf! :)
Now, onto the things I have to suggest. Keep in mind that this is my opinion and whoever disagrees should take it with a grain of salt and everything that is written is pretty much lackluster.
Personally, I have two major gripes with the current state of balance. Those are Energy management skills and the Legendary system.
First, I'd like to talk about Energy management. It feels that with certain skill and core set ups that players have ability to impact another players Energy way too hard, and in some cases their own. This in my experience and builds holds a major issue due to my tendency of maxing a certain skill and relying on it in order to win a duel. In cases that I end up dueling a player who has a highly scaled or leveled skill that drains my Energy and/or cores that drain Energy I end up losing by default because I don't run the same skills and cores in order to combat the other player in his own game. Thus rendering a build like Maxed Massacre relatively useless. Since it becomes my most reliable sort of damage and win condition. To me it feels forced to rely on Energy management skills and cores in order to have a valid chance of winning a duel between even opponents and it takes out quite a bit of creativity in builds because it becomes as mandatory in builds as were old passives.
For this specific issue, my own suggestion would be complete removal of cores (I will have a take on cores later on in the post) that affect both the player and opponents Energy and balancing all skills that drain and gain Energy around a smaller number with a mediocre scaling. I belive that this would have a positive impact on resource management balance, while also providing support for builds that are oriented on higher Energy consumation and builds that specialize around Energy skills. Because by limiting how much a player can impact an opponents Energy will provide more reliable Energy pool that would allow certain builds to have an impact, while also making builds that are specialized in Energy skills playable by providing scaling to them. In other words, I think it would make both playstyles of a one trick pony skill and energy looper realiable.
Next up, the Legendary system.
I belive that a system that provides players with additional damage and/or tankiness has no place in a player versus player oriented game. For this I will take League of Legends' runes and items as an example. In LoL the runes provide players who meet certain conditions a burst of damage, amplifiers of damage, tank stats which also deal damage and with items like the duskblade which under certain conditions reward players with an additional burst of damage. This brings a damage creep into the game because everyone will deal more damage with easy to meet conditions and in some cases be more tanky while also dealing more damage. The reason why I'm tieing this in with the Legendary system is because it promotes unhealthy balance issues. In EpicDuel the condition that a player has to meet is experience, in other words, if you grind you become stronger. While I do accept that some progression in power is needed I belive that it shouldn't be tied to a system like this. Because a player who has the free time to play 24/7 will always be stronger then a player who has two or three hours to spare a day even though they are at the same level. Personally, I very much liked how the old leveling process went. You level to 30 and play an even game until an additional level gets added, then all the players grind another level and play an even game again. For players like me that can spare a couple hours each day it feels extremely unbalanced to play againts someone who is a hundred Legendary ranks above me. And it demotivates me from playing the game because I am fighting againts someone who has a lot of additional power for 90% of the battles on the same level that I am.
For this issues I would suggest making the Legendary system only as a vanity system. Because with a removal of legendary progression it would provide a more even playing field amongs players who reached the maxed level and remove the stat creep it provides.
Now onto stats and skills. Honestly, both feel weak. Skills seem like they aren't impactful enough and stats seem not to provide much. In order to keep this shorter, I'd just suggest balancing both stats and skills in a way that they provide more impact in battles in a way that is balanced.
Speaking of stats, I think that the idea of each piece of weapon, armor and other items providing different stats was great. It made a lot of diversity in builds, especially those that weren't the flavour of the month ones. I belive that having every item have unique stats would bring back a lot more creativity and fun in build making, thus making each battle feel more unique, different and in turn more fun. While I do agree thats it brings an RNG aspect to the battles because no one is gonna run the same set up, I still think that it would be much more fun compared to everything being anything. (I belive that I still have the google doc with each items stat bonuses up until a certain point)
Also on the topic of stats. I actually think that it was much simpler and fun having the Health and Energy pools be 10x smaller. I loved to calculate my damage rolls with the addition of them being circled into a higher roll or a lower one. With higher numbers my calculations became much less reliable because I'm not a mathematical genius so that I eventually just stopped caring about it and just rolled with whatever came up on the screen. Also it looks much, much cleaner and to me it was easier to read the smaller numbers.
Now onto cores. Cores, well I never liked the feature, nor I didn't like it. It kinda felt meh for me. Some seem to be extremely powerfull, while others not so much. I personally liked the system where my Delta weapons would have a 13% to curse an opponent, rather then the one where I need to spend 130 Energy in order to curse one and lose a turn. Some cores like the curse one just feel like they don't belong in the current state of the game. I mean its great if I run into someone who has 100+ support, but whens that gonna happen now. It kinda ties into the older RNG aspect of the game. While some of the RNG was too much, some of it was good for the game and made it more fun. Honestly, I have no idea what could be done to cores to make them feel more enyoyable other then tieing actives back to RNG on-hits again and letting passives be passives.
And lastly I will write a bit about passives. Passives were indeed far too powerful, but I never saw enough justification to their complete removal. They were mandatory in every build and all of the builds were revolved with them in mind. I'd love to see them back again, because I think that even too much skills can be a problem. It just felt more realiable having them present all the time, rather then being present as an active skill which provides nothing more then a strike and a buff towards %Damage healing or Energy gain, whichever. Also boosters. Health and Energy boosters were great.
Feature wise I really don't have much to suggest other then adjusting currency gains a bit.
That would be all from me, and once again this is just my opinion.