If you do not do a pacifist run he will be the "Glad Dummy" on genocide so do pacifist.

Stock up on lots of food items. Do not eat the Butterscotch Pie EVER YOU WILL NEED IT.
If you already have eaten the Butterscotch Pie, do not fret, there IS a cooler before you see the dummy. Go up to it and interact, there you can take up to 2 or 3 Astronaut Food.

Ok so you want to have the Mad Dummy attack. At first he will have dummies come up and shoot these little cotton ball things at you. If you dodge them, they'll come up and hit the Mad Dummy, and it will cause a little damage. After he says "Relying on people that aren't garbage," he will start using Dummy Bots which will shoot Magic Missles. They will follow you until the red dot is completely gone, so just circle around a few times, making sure they hit the Mad Dummy. After this, he will throw a single knife at you. Simply dodge the attack.

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FrostyFlare helped make this guide.