FULL SCREEN RECOMMENDED Controls: -Arrow keys -Jump with up -Select/next speech bubble with Z -Deselect with X -Red heart: move freely. -Blue heart: jump physics. -Don't move when touching blue attacks. -Your items get refilled after the halfway point (Sans sparing you). -This project needs a pretty good computer to run and can get laggy at times, especially when using Gaster Blasters, so if it seems laggy, try downloading the project into offline mode so it runs of your computer instead of the internet.
For those of you who either don't have the game like me or are just too lazy to play the game to the actual fight, this should help. All credit goes to @loganvcairns and Toby Fox.