Whether you can stack a skill or not is never important at all. The essence of the fight is simply 'outdamage her regen'.
What you should consider first is your dps. This can be split into 2 parts, total damage per attack and your fpa.
Therefore you have to look for a skill with huge total wdm per attack. That kind of skills usually multihit a lot so it's likely to have people think that stackable skills are the key but this is actually a penalty to those with big damage, especially against teleporting / moving bosses which occupy 90%+ of your bossfights in MXL. At this moment the combo with solitude takes a big part of the fight though.
Theoretically the newest patch (deimoss avoid) should decrease those multihitting skills' dps against her by up to around 40% (disputable, depends on the number of hit per frame.) but there's a bug in the arena, related with her regen so I can't tell you the exact cap of your dps to do the quest. If you feel like she's doable with previous dominion builds then it would prob be due to the bug..
As I've mentioned a lot in the discord channel, there are five possibilities I've found among wdm skills.
Disco inferno isn't restricted to a specific weapon type and it's easy to aim but has the second lowest total wdm, and also consumes huge mana.
Stacking +1 MoS Relics for mana sustain doesn't work anymore due to the latest change, avoid.
Wychwind has the highest total WDM against mobs with big hitbox (except for tantrum with unrealistically stacked amount of relics), and the highest hitrate.
Deimoss has a small hitbox and the chance to activate her avoid scales with the number of hits per second now, so Wych lost lots of damage in 2.2.
Don't forget that you need a throwing knife for Wych, which has bad damage, bad pool of weap choice and bad fpa with the most of chars.
Magnetic field requires a scepter and hits 100 times on its center. A scepter is not a good base but at least better than a throwing knife. And it has 100% conversion which counters monsters' block (ex - phoboss). The good point, Mag and Tantrum can hit deimoss and her adds at the same time.
If you wanna build a deimoss killer around magfield, then zon might be the best choice because thunderbane's bonus damage for magfield is really huge
(TY to Ben for the tip but stormzon sucks tbh)
Tantrum requires a bow or a xbow. Overall the best skill at this moment. Stacking relics greatly increases your dps via the Petulant and the duration.
The highest total wdm per attack, some great bows/xbows like Valk Prime, Stygian Fury or crafted ones.
Sky Siege also eases the difficulty of the fight by a lot cause you need less time to chase her mobile adds and to dodge her attacks.
I believe this skill should be nerfed hard, it's prob the only skill among these that shows outstanding performance in ordinary farming zones.
Twisted Claw has the lowest total WDM. But it works with melee devo and hits the least so it won't trigger the avoid often. The only chance to do her with Tclaw is Azgar wolf imo. You'll need to do super good min-maxing but yeah it's possible.
As a caster it becomes much harder, you need relics for a solo kill. There's another strategy that take advantage of the 6 shrines as a target of an auto-aiming skill. For instance, if you place a voidstar right on her head then it will shoot 4 more projectiles to the shrines and adds which deal the damage to Deimoss as well.
Ember Spirit is hard to aim and insanely multihits, Idk if there'd still be a chance for ember du even with stacked buff relics.
There are a few more possibilities but I actually haven't tried those ingame nor did hard to theorycraft and minmax a build.
If you have a group to do the quest together then it'll be much easier. You don't even need relics or tons of umos. In 2-3p you also have lots of choices. A player should go into the arena alone after opening a portal and let others join the game and come into the arena a few seconds later, deimoss will have the same hp as 1p game (her adds not.) You guys may take a supporter for buff/debuff, 2-3 dps also possible, gives variety to the fight but unfortunately it's too hard to make such a group with experienced players who have the same goal and motivation to directly rush to the end of the game.
Another main point is to lure his ice mist (Misery) which fks your FHR and deals tons of dmg. It has 45sec cooldown so you have to avoid standing near her (or the edge of the arena where you're supposed to do the damage if you're ranged) when she's ready to cast it. Especially for melee builds it's gg if she stacked 2+ mists around her.
About tank I can't really tell much. It varies a lot. A char with big dmg like Necro can still run a glass-cannon build in her arena but it'll be a bit more risky and fishy, bad rng may completely ruin your challenge. Insane tank of some builds like azure disco require a bit less dps cause he doesn't have to dodge the most of her attacks and her adds'.
Still waiting for another creative kill.
To people who want to try her, I'd say just go ask a lot. I mostly answer all the relevant questions via discord. Arcology, Ben and Mortimer did the same thing for me. Players with enough knowledge like Ban- would never hesitate to give you a solution if you hit a wall. You can take a look at my / other players' armory. Just try to avoid too general questions like 'which gears do I need to kill her?' 'cause you can type it in 5sec but they'll have to spend 5 hours to give you an answer. If you wanna copy paste the complete build with all the details without thinking at all I would not like it because that'd insult all my efforts spent on the build, however you can even do that if you want.