The genocide route is much faster in terms of earning trophies, but it's going to be a lot harder to complete to the end. There are consequences that occur if you finish a genocide route that affect all future pacifist routes until you manually delete your save data, so this is best suited for people who have already seen the pacifist ending. This run is written with the assumption that you've finished the pacifist or neutral route.
If you remember how the game starts with Flowey, see what happens if you decide not to accept his little friendliness pellets. You're out for blood, remember that. When you fight the dummy, do whatever you feel like, but it'd be a good idea to get used to attacking. The first Froggit you encounter can be killed in one hit if you properly time your attack so it hits the middle of the attacking reticle. If not, no big deal. Continue through the ruins until Toriel lets you go on by yourself, then camp out at the save point and pick up the candy to the north if you'd like the healing items.
Save at the leaf pile save point and walk around in this room until you kill 20 enemies total; you'll know when you hit that amount. As you kill more monsters, your EXP and LOVE will increase, making it easier to kill them faster. Whimsun are the easiest things to kill, so you'll hit 20 kills faster if you happen to run into more of them. Once you've killed the 20, you'll now find that your stats have a kill counter in them.
The ruins will be almost entirely empty. Continue through the three colored button puzzle and, before you reach Toriel's house, continue right to find the toy knife. This will increase your attack power. Sleep in the bed in Toriel's house for her slice of pie and confront her at the door leading outside. Start by attacking her and then leave towards Snowdin.
After your first interaction with Sans and Papyrus, you should soon find that all of the puzzle-solving leading up to Snowdin has been cut out. There's no need for puzzles when you want to murder everything you can. Take the tough glove from the storage box and equip it for +2 more ATK from the toy knife. When using the tough glove, press four times after landing the attack so you can actually deal damage. You can start working on the 16 enemies you have to kill in this area right here from the room with the storage box in it.
Note: Snowdrake must be killed in order to continue the genocide playthrough. If you run into Lesser Dog, it has to be killed as well.
When you reach Doggo's outpost, he'll start to shiver for some reason. Killing him is mandatory. Up next is the first patch of ice you're able to slide on, and to the north is a snowman. Take as much of him along with you for healing items as you like. When facing Dogamy and Dogaressa, kill Dogaressa first; this is because doing so will make Dogamy weaker, and killing Dogamy first will make Dogaressa tougher.
All of the puzzles are deactivated or can be ignored. Glyde can be encountered if you find him before facing Papyrus (and if you still have some enemies left to fight), but it takes longer to make him show up in his room when you have more kills. You'd probably have to keep moving in the room for a good half hour. It's not really worth the trouble since you don't have to kill him anyway. You do have to kill the Greater Dog, which is just past Glyde. Try to wrap up your standard enemy kills before entering Snowdin Town.
The shop owner in Snowdin town will not be present if you've finished your 16 kills. You can steal the money and as many of her items as you'd like before looking around; make sure to definitely pick up the manly bandanna, then fill up the rest of your inventory and the storage box with healing items as a backup.
Note: You must finish your 16 kills before fighting Papyrus, or the run reverts back to a neutral playthrough.
Papyrus is an easy encounter, to say the very least.
Waterfall's kill counter you have to reach is 18, which you can still check at save points. The rocks will not fall from the waterfall that they usually fall from, so it'll be easy to get through the water and pick up the old tutu for +3 more defense from the manly bandanna. Shyren must be killed here. You should be able to start killing once you reach the second room with bridge flowers, but you should hold off until after Shyren automatically encounters you near the piano that opens up the path to the whole dog residue ordeal.
After falling off a bridge towards the dump, you'll find the dog shrine's donation box. You'll be able to earn those trophies here if you haven't already, and doing so should be easy, especially if you stole from the Snowdin Town shop. Kill the dummy once you reach it.
Gerson is willing to sell to you and talk to you since he knows you can't kill him from the shop menu. Try to finish your 18 kills in the area you have to light up with mushrooms and lamps, then backtrack to a save point and save to restore your health - and to make sure you don't have to kill any more enemies again if you lose to Undyne.
Undyne is much tougher here than during the pacifist route due to her remarkable determination to protect everyone. Her attacks are wild from the start and deal a pretty scary amount of damage. You ought to use for this fight for more control, and remember that practice makes perfect. Her attacks she does when your SOUL is red are usually the most devastating, especially the circling spear ones.
When you eventually get through Undyne's health, she'll tell you some info that spells out impending doom for you.
There are 40 monsters you need to kill in Hotland. This is a very slow grind in Hotland's main area, so it might be worth it to leave most of the grind for the CORE. You should eventually find the two royal guards, both of which are easily killable (and must be killed). The elevator can reach almost all floors instantly, so go ahead and ride it to the top. So Sorry cannot be encountered on the genocide route.
Muffet can be killed instantly. Catty and Bratty aren't at their shop, which means you can take from them as well. You don't really need money, but they do have some you can take. They have a new weapon and armor that you can take as well (not that you need them for Hotland). Burgerpants will be at his job if you'd like to buy from him. The steaks in the shape of Mettaton's face is your best choice because it heals 60HP, with your second best choice being the legendary hero (a sandwich in the shape of a sword). You're definitely going to want as much of these as possible.
The CORE has more enemies per battle and has scripted battles against seven total enemies on the northern path. Finish your kills and take the elevator at the start of the area to get to Mettaton's room instantly. Taking him out requires no new information, just like most boss encounters so far.
When you arrive at Asgore's house, take the locket and a real knife from presents in one of the bedrooms (the best weapon and armor in the game), then take the keys and unlock the chain. You should now be Lv19. Trek through the castle until you reach the Last Corridor and face your judgment. Make sure you're using the new gear and have your food from Burgerpants in your inventory, then save your game.
Sans will judge you once again, but it won't take as long. Being the violent person the fallen human is, it'll be time to fight Sans. Sans only has 1 ATK and 1 DEF, and is the easiest enemy. He can only deal 1 damage. There's no information anyone could give on this fight that would help you get through it aside from two simple, obvious things:
- Good luck.
- Practice makes perfect.
There are videos of people performing a perfect battle against Sans in which they take no damage, and there might be an online practicing program to help you as well.
The ending to the genocide route will play out without any more battling on your part after beating Sans. If you really, really, really are having a bad time trying to beat Sans, I guess you could look up how it goes for yourself, but that's no fun.
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