I am currently doing an Undertale Genocide Route and I am in Hotland/Core, just before fighting Mettaton. I'm trying to hard to find the final monster for me to kill but I don't know where to look. I need help trying to figure out how to find this one last monster because I have been wondering about for hours.
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.
On the bridge right next to Mettaton NEO, walk towards the north part on the bridge. Once you hit the north wall, hit arrow UP and arrow DOWN and hold it. You will get the last couple encounters. Hope this helps people.
Note: DO NOT GO INTO METTATON'S FIGHT ROOM UNTIL YOU EXHAUST THE COUNT OR THE GENOCIDE ROUTE WILL BE ABORTED!
In regards to Hotland/Core, the last three monsters are actually not random encounters. The last three are in Core. I can't remember EXACTLY where it is in Core, but you will get a mini cutscene of a silhouette of the monsters and then get to fight them.
I'm not sure if there was a patch to change this, but as of April 4th, 2016, this was the way I had to kill all of the monsters in Hotland/Core
EDIT: The spot where you find the last three is the same spot as the three monsters on the bridge in the Pacifist run, which I believe is the very north part of core
EDIT 2: I just did the genocide route again last week and this was how to get the last three. So I am unsure how this could be wrong.