You're welcome ^^
I know for a fact that it's possible to one-shot without a tao.Peco Peco Egg Robot Arm would be the best weapon, Glorious Suit, and Meg would be the basic things you need for this. To boost damage further you would need Temp Vit, Giant Faceworm skin, and if that is not enough, Turkey on Head.
Once you reach around level 160, you should be oneshotting. Like what others have said, use +20 vit and str foods, HP Increase. I also would recommend using Honey and Sesame Pastries to boost flee and hit.
Edit: - Oh and use Bloody Murderer trans candy for that 1k HP boost
Robot Arm has 2 slots, he may use one Pecopeco egg card and one Big Bell Card + Neo Punk Card on Shield for have its bonus of +50% dmg to Formless. in total will be +80% dmg on formless monster (20% from pecopeco egg, 10% from Big Bell and 50% if big bell and neo punk are compounded to their respectives equipments).