People claim to have pulled it off but I have no idea how they did it. I've tried on a fresh savefile not 20 min ago to pull it off but still nothing.

The only way I've been able to progress past the Julia event is to swap bodies while not having saved Ashley fully. Immediately after the swap (while still Julia), go back to your house. You then just have to do absolutely nothing in the library waiting for 5 or 6 scenes until you get swapped back and chaotic events happen (While you can try to leave, the game will not let you and my best guess is that the timer to the next scene resets). You can then go to floor 24, get a scene and continue to floor 25.

I did try this same scenario after fully saving Ashley and doing all Julia dates, but the scene plays a bit differently and I couldn't get it to progress. I also want to mention that I didn't actually complete the heroic ending because near the end I talked to Val at school and there was a choice option that softlocked the game and my save was right before the Julia event and I haven't played it back yet.

Potential other requirements to progress past Julia event this way: My lowest floor was 12. No one was charmed; I was Rosa (lvl 21), My teammates were Val (lvl14 + dating), Sarah(lvl14 + doll + House scene), Cassandra (lvl14 + all Sarah scenes and went on date), Molly (lvl 14 + quit job through persuasion after 3rd date), Ashley (lvl 8 + Jane helped clear mind but haven't done contract scene). Kate was in the plaza (I was waiting to tell her that Brittany was an alien), Brittany hadn't been met yet, and I had done the first two monster capture quests and went on a date with Erin, unlocking the monster ranch scenes for naga and mature slime.

Okay, so good news and bad news.

Good news is, I found out a way to get to the Heroic Ending. 

Bad news? It does not work - at least if you're dating Sarah. I can't vouch for other characters.

Do the Heroic route as usual but NEVER grab Ashley. Yes, I'm serious. You may not grab her.

When confronting Julia, push past her and beat her dungeon.

Once done, do NOT speak to Dr. Tiff. Instead, head home and meet Julia. 

Wait in the library for all scenes to complete (the last one involves a monster appearing in the library). Once done, head around town. 

I recommend going to the protagonist's house first so you can heal (also unlocks Val).  Next is up to you, but I prefer this order:

Head to the dungeon (1st floor) to get Sarah.

Head to the Item Shop to get Molly and Derrick

Head to the Town Square to get Kate - Enchantment Shop to unlock Brittany's enchantments again if you don't have them already.

Head to the Lake to get Reese 

Head to Drape's room to get Cassandra 

Next, reach floor 24 to fight a (rather tough) boss fight. Once done, head out and head to floor 25 (floor 24 sigil doesn't work). Head to the depths of that floor (hint: take the right path) for another boss fight. Again, this one is bugged so one of the options doesn't work. So you'll have to beat it. 

Head out and speak to Angie in the square. Choose any option when prompted.

Head back to the school and lose the battle. Watch a scene and... yeah, Val's in the hallway but you can't click on her.