Oh yeah, I figured it out, with the right combination of buffs and equipment it's fairly consistent, I actually think it's easiest to do with Dante as then you get to use Keke's claw weapon, which attacks three times in a burst and has slightly higher combined base damage than other weapons at that point-
Basically the strategy is-
Soften her up with Bessie, use all the available buffs in base form like cheer, feeling lucky, etc.
Use Ray for healing and nulblaze, and get 4-5 stacks of cheer on Dante right before you're ready to attack, then taunt using Dante to raise his counter chance, and attack, you'll make her rage with your attack, but you'll ideally either kill her with one triple attack, or you'll kill her with a counter, since she ALWAYS attacks you before she heals or buffs herself when she enrages.
My party WAS level 16, but that's not really out of the ordinary if you're collecting legendary equipment like you should, and doing all the quests, I didn't even do repeatable quests, I just got the equipment I wanted to farm and moved on.
The reason why I was thinking of a way to do it as soon as possible is because, frankly, the scene is cool and easy to miss as you'd have to replay the whole story mission from scratch if you wanted to fight THAT specific version of the pumpkin queen, so it saves time, plus the Pumpkin Slayer really gets outshined by even basic weapons from the following areas, so if you don't grab it then, you'll basically never use it for the rest of the game-
Heck, it gets easily outclassed by Bessie's most basic shotgun literally as soon as you move on to the village. So it's mainly just for completion, bragging rights, and hinting at what's to come.