How to find the last monster in Hotland/Core?
The problem is that the more monsters you kill, the less frequent the random encounters are. This can make the last couple very tedious. However, there is a cool hack/trick that I have found to help you get the last one.
Go to an area that monsters definitely can spawn in. They usually don't spawn in puzzle rooms, so just go somewhere that you remember having a random encounter in.
If you are in fullscreen, hit
F4to go back to windowed mode.Hold down an arrow key. Any arrow key, doesn't really matter, but don't walk into a wall or an object.
While you are still holding down the arrow key, press alt-tab. This will cause the game to lose focus. However, Frisk will continue to walk without actually moving. The game registers this as you walking.
Watch youtube until a random encounter happens. This shouldn't take more than 2-3 minutes at most.
This doesn't make grinding very much faster, but it definitely makes it less tedious.