Undyne is the third major battle of Undertale, coming thick and fast after exploring Waterfall and Temmie Village.
The following boss is designed to be played as part of a complete Pacifist guide - in short, going through the entire game without killing anything to unlock a different ending - of which bosses are some of the hardest to beat.
However you're playing, our wider Undertale walkthrough delves into other areas of the game.
Undyne boss fight
There's no Act you can do to help with this fight - on every turn you need to just attempt to Flee.
When the fight starts Undyne will turn you green making you unable to move during her attack phases. She'll then fire spears and arrows at you from various directions, and you need to deflect them by facing in the appropriate direction.
The attacks will speed up as the fight wears on, and eventually 'trick' yellow arrows will appear - when they get near to you they'll loop around and attack from the direction they're pointing rather than the direction they approached from.
After a few rounds of this she'll turn you red and attack with spears before immobilising you again.
Once you've managed to flee, make a run for it. She'll catch up with you a couple of times, but just keep fleeing and eventually you'll lead her to Hotlands where she'll collapse from exhaustion.
If you're going for the Pacifist ending you must quickly grab a cup of water from the nearby watercooler and pour it over her!
Playing Undertale? Our complete Undertale walkthrough and Pacifist guide will explain how to finish the game while sparing everyone, covering all areas including the Ruins, Snowdin, Waterfall, Hotlands and The Core and more. You can also learn about how to unlock every Undertale ending and all Undertale Trophies, access hard mode and play the game in the most evil way possible with a Genocide run.
Once you're done and you've managed to get through the battle both unscathed, you'll be heading to the Hotlands - though there are one of two things before moving on.
Head east from where Undyne fell over, then make your way south and ride the ferry back to Waterfall to meet her at her house.
Papyrus will be waiting outside - speak to him and head into the house, and he'll beat a hasty retreat.
Take a look around if you want, and then sit at the table and she'll offer you a drink - when you try and get up she'll get a bit overexcited. Point her spear at the tea on the counter, and she'll then offer you a cooking lesson.
Follow her instructions, and eventually you'll end up in a "fight" with her - just Fake Hit her to end the battle.