Muffet is arguably one of the more difficult bosses in Undertale. Especially if you’re going for a Pacifist run – which means not killing any creature you encounter. In doing so, you don’t get any loot or EXP, making any traditional battle very difficult. Muffet takes many rounds to pacify, and her attacks are particularly erratic and challenging to dodge. Luckily, there is a simple trick to make her drop the battle altogether – in a single move. Note that this strategy requires making moves early in the game. If you’re already at Muffet’s door, it’s still possible, but it’ll cost you a lot of time and money. 

In the beginning hours of the game, you’ll find yourself in the Ruins where a quaint Spider Bake Sale is offering a special Spider Donut for 7g. Purchase this item and be sure not to eat it by keeping it in your Dimensional Box until you approach Muffet’s lair. Then, simply add it to your inventory and give it to her in the first round of combat. She’ll immediately end the fight, touched that you are really passionate about spiders. That’s it! 
If you chose to skip the bake sale in the Ruins, you can still purchase the donut from Hotland. However, it’ll cost you 9999g. Only a mere 9993g markup! (Or, you can use Flowey’s Time Machine.)

Check out the video below to see the Spider Donut strategy in action.