edit: Nami noooooooooo! haha
There are obvious ways to make the current design slightly less terrible, like each person can only use sword once, etc. that would be more in line with the "random Atheon teleports" philosophy. But the current encounter is fundamentally broken, it would need to be rebuilt from the ground up.
That pre-release image of Scrota in the darkness surrounded by beady eyes was a better starting point. An enclosed arena battle with a more reasonable sized boss who doesn't insta-kill on such a tight timer but has more give and take with sword attacking, closer to a Souls boss. Needs way, way less focus on DPS gear checks, more dynamic movement, more opportunities for varied roles between teammates. Needs to actually use the "building a vocabulary" philosophy of VoG so mechanics introduced early on are combined together to test your understanding.
1.) Sword is cool but needs way more of them so other people can have fun too even if it's just controlling adds. Flying around chopping >>> standing still sniping. Timer is fine but would need to be much longer which would need to be balanced with doing much less damage per hit on Scrota.
2.) Presence of Scrota is fine, you could have dedicated healers i.e. orb generators for example. Key here is to throw this Lv33 forced under-leveling garbage out the window, it's the most critical design failure of HM CE, need way more give and take with enemies damaging you. Same as above: re-balance sword damage so having players at the same level as Scrota doesn't break the fight like now.
3.) It's dark, so use the lamps!!! Have to activate them to be able to see anything, multiple lamps required for maximum visibility: think Link to the Past final fight vs. Ganon. They're on a timer, same as abyss, but could be re-lit somehow (ideally a new mechanic, a sword ability similar to VoG relic cleanse). If they all go out it's literally pitch black: that would be fucking intense.
4.) Of course this mechanic would naturally tie in with weight of darkness mechanic. Not only would you lose visibility but also mobility if the darkness grew too strong. The lamps would light the entire arena to remove darkness (i.e. ALttP), not exactly the same as standing next to them in abyss but similar.
5.) Lamp lighting could be combined with bridge crossing. Need to kill sword bearer to access side areas where lamps are. Have to kill GKeeper to activate lamp or/and re-light a burned out lamp. Or maybe kill Shrieker to break wall to access lamp so teammates work together w/ sword crossing and shrieker blasting.
6.) Wizards form vestige pools if you don't kill them quickly enough so this would be one of those snow-ball effects. They spawn when a sword bearer dies so someone would need to take sword while one or more control wizard(s). More swords would mean more lamps and attacks at Scrota but also more wizards so it's a balancing act. Couldn't trivialize wizard spawns by not killing Boomers like now. They would have to spawn because you have to get swords.
7.) Oversoul is okay but needs to be much easier to kill so it's less of a DPS check and more so another thing to manage during the fight. Enrage should be similar to Templar and Atheon where it's still possible to finish off the boss, not the insta-wipe nonsense it is now. Maybe Ogres start spawning then but makes no sense to have perma-Oversoul, unless DPS was lowered a lot.
Multiple sword bearers so everyone can get a sword if you kill them fast enough, vestige green circles on the ground to encourage airborne sword mobility, still need to control adds but in a more free-flowing manner similar to Templar instead of Tower Boomer silliness. That gives everyone a lot more to do and requires a lot more constant communication.
It's silly to be an "armchair game designer" but these are all mechanics that exist in TDB, not asking for tons of new stuff to be introduced that would require production time. The actual balancing of all these elements would be very difficult, but that's the point: complexity, not something that feels half-baked like Scrota now.
