Rather than talking about how to use her, lets talk about how to fight her instead. 

First up, she caps all other units speeds to be at most 90% of her own. This does not mean she makes their speed 90% of hers if they're super slow, it just means they won't be able to pass that 90% amount. You'll typically find her built base speed right now, which creates a ton of RNG because there's no way to control who goes after her. But you shouldn't assume that she's going to be base speed, because some people run her at faster speeds which ruins anyone trying to bring full base speed units into her. The only way to take first turn from Harsetti right now is Zio, since he gets 30% CR at the start of combat. Zio into some other cleave units like Jack'O, Blooming Lidica, Requiem Roana, and Summertime Iseria is a potential option. Aside from that you mostly just want to use a bunch of tanky units. She also prevents CR pushes on her turn, which shuts down most of the common cleansers like Designer Lilibet, Elena, or Infinite Horizon Achates. Elena after her buffs on 10/24 will be able to cleanse with just her S2 as long as she doesn't get stunned by Abyssal Crown though.

My preferred method for fighting Harsetti is by using Blood Blade Karin, Amiki, and Abigail. BBK in the back slot lets Abigail cleanse her when she drops to 1 HP and gives her a 3rd immortality. Harsetti is likely to strip the one BBK starts combat with, so this guarantees her 2 Immortality via her EE. Amiki already has self-cleanse and can get 2 immortals as well, just watch out for CD increase skills and her getting stunned before she can use S3. If your Amiki takes a turn and hasn't procced her S2 yet, just use S1. The 4th unit slot I usually use Arbiter Vildred into tankier teams like Harsetti, ML Senya, ML Ilynav, ML Yufine / BBK. If it's a debuffer team like Harsetti, ML Romann, BBK, ML Yufine / ML Senya / ML Ilynav, then I use ML Senya instead. If your Amiki gets procced before Romann moves, make sure to kill him first so he doesn't strip BBK. This team is very consistent for me and I haven't lost since I started using it.

One other team option is Aria + Christy with a cleanser and flex slot. The Cleanser can be Maid Chloe, ML Kawerik, or Laia. Flex slot can be Elvira for fighting ML Yufine, ML Ilynav against ML Senya, or a bruiser like ML Landy or ML Bellona. This team runs the risk of fighting a 300 Eff Harsetti that debuffs your Aria and then bad CR rng makes Aria go before your cleanser, especially if you're autoing.

An older team I stopped using is Light Angelica and ML Tywin to cleanse, ML Cermia as the DPS, and ML Ilynav or Roana. There is a bit of a risk involved since if Harsetti has Crown, the extra attack will proc LHC S2 then stun her before it actually activates. So there's a chance LHC doesn't really get to move. Besides LHC, I've tried using Blood Blade Karin with a 2nd Def Breaker like Festive Eda or Briar Witch Iseria. This worked ok but was pretty sad when I missed Def Breaks.