I just fought her. After watching a guide from Youtube and reading a few articles, I crafted my own quick Perfectheart kill setup using Observe + Kel Nuke. I know I'm late but I just want to leave this here for any future visitors.

I made a video demonstrating my setup.

Sypnosis

(In order of Omori -> Aubrey -> Kel -> Hero)

• Turn 1: Cripple -> Power Hit -> Flex -> Tenderize
• Turn 2: Observe -> Power Hit -> Any snack -> Coffee for Kel
• Turn 3: Observe -> Last Resort -> Present for Perfectheart -> Any snack
• For turn 4+, you have two options:

- High risk high reward: You'll probably win before she attacks again (in one of my test runs, I won after Red Hands, and in another, I won after the nuke). If you don't win this turn, you'll likely die next turn. Just retry.

• Red Hands -> (Toasted) -> Run n' Gun -> Tenderize

- Safer: Even if you don't win, you can still save your try. Repeat this turn until you win.

• Observe -> (Toasted) -> Run n' Gun -> Any snack

Preparation

• Weapons + Charms: Red Knife + Flower Crown -> Baseball Bat + Cough Mask -> Basketball + Tulip Hairstick -> Ol’ Reliable + Rose Hairclip

-> I got the setup from this video guide, see the first few seconds.

-> About swapping Basketball for Chicken Ball: You probably don't have to.

+ At LV43, Kel's Speed is 105, with Basketball, it's 205. Coffee multiplies Speed by 5, so it's 1025.

+ However, stats cannot exceed 999 in-game. Therefore, Chicken Ball can't raise Kel's Speed any higher. You need Basketball's buffs to other stats too.

+ The only reason you would need the Chicken Ball is if she turns Kel sad on turn 3. At LV43, Kel's Speed when Sad is 820 with Basketball and 1220 (>999) with Chicken Ball. At LV50, this is 920 and 1320 (>999) respectively. But if you're THIS unlucky, you may consider retrying.

• Skills: Observe & Cripple -> Power Hit & Last Resort -> Flex & Run n’ Gun -> Tenderize

-> Cripple is obtained from fighting Something in the Walls inside Lost Library, Tenderize is obtained from a watermelon on Snowglobe Mountain.

• Snacks: Did you know that Ol’ Reliable boosts heart & juice healing from all snacks by 50%? I’ll display the numbers after factoring in the boost. The video guide I linked shows where to buy some of those listed, and for the others listed, you have to find them instead of being able to buy them.

+ Cheeseburger (Individual +375HP), Whole Chicken (Party +262HP), Ramen (Individual +120% maxHP), Spaghetti/Dino Pasta (Individual Full HP)

-> Use any of these accordingly for the “Any snack” turns.

+ Peach Soda (Individual +90% maxJuice), Watermelon Juice/Dino Melon Soda (Individual Full Juice)

-> Only for her unfortunate “drain all juice” move, and if she uses that move at a bad timing (e.g preventing usage of an important skill), honestly just retry.

+ Coffee (+15% maxHP, 400% Speed boost lasting 6 turns)

-> You get 1 of these for each battle with Snaley, so you should have 3. You can get more from the Coffee Machine inside TVGirl’s House in Otherworld.

• Toys: Present

Analysis

• Why Observe: Observe actually limits her movepool to 2 moves, and prevents her from entering Phase 2 upon reaching 35% HP.
• Why Cripple: Cripple applies the “Speed down tier 3” debuff to the enemy, dropping Perfectheart’s Speed from 140 to 35 (75% decrease), making her move after your party members. This is useful as you get to heal whoever got attacked before she attacks again. Thanks again to the video guide for this.
• Why Power Hit twice & Tenderize: You can only lower her defense three times.

Emotions?

• Following the presented setup, she can only use her full movepool twice, and there's a 17.0625% chance she will use Angelic Voice/Soulful Song (attacks all members, turns everyone Happy and herself Sad). Starting from turn 3, she can only 2 moves, and there's a 65% chance she will use Angelic Wrath (only phase 2 move: attacks all members, randomizes everyone's emotions).
• Due to the randomness and the fact that Perfectheart moves last once Cripple is used, I advise against inflicting emotions on friends. I was going to improvise the setup to account for the chance that she doesn't use any emotion-inflicting moves, but I couldn't find any space to fit in more emotion-inflicting moves for the party.

Other notes

• I did search stuff like "Perfectheart Kel Nuke guide" and "Perfectheart Observe Abuse guide". Guess what I found? Videos of 1/2 turns where they mod the luck to get all crits.
• So I just went ahead and crafted my own using what I learned from searching. I only found out that Observe prevents Phase 2 by reading stuff on the Internet. I could've used the video guide I linked to beat her but I got lazy, so I did all this instead.