Ruins- Kill the first Froggit (the one that Toriel shoos away), and then exclusively Loox. Pick On him 3 times, then Don't Pick On Him 4 Times, then Pick On him 3 Times again (You might only need 2, I'm not sure). Picking on him increases the EXP by 5 if the current yield is less than 20. Not picking on him reduces it by 2. He starts out with 7 EXP. 7 + 15 = 22, then 22 - 8 = 14, and then the last round takes it up to 24. Once you get to 19 kills, make sure your last encounter is a double Loox, and do the same thing to both. While you're in the ruins, if you're saving the Snowman Pieces for Sans, like me, Spider Ciders are the second-best healing Item before Undyne. If you somehow don't get enough money before leaving the Ruins, Cinnamon Bunnies heal 22, which still isn't that bad. You should be at LOVE 8 before Toriel

Snowdin- Make absolutely sure you kill Snowdrake. If you don't, you will be thrown off the Genocide path. Kill the entirety of the Canine Unit, but if you don't encounter Lesser Dog, you don't have to fight him. Unfortunately, he's guaranteed to be your 3rd encounter but thankfully/unthankfully, you can farm Glyde. for some reason, you can kill Glyde infinitely, meaning 100 EXP until Snowdin's kill counter is exhausted. Once it is, continue through the rest of Snowdin normally. After Papyrus, you should be at LOVE 12.

Waterfall- Don't forget to grab the Abandoned Quiche on your way through, as it is the best healing item before Undyne if you're saving your Snowman Pieces for Sans. The area is pretty easy compared to the others. Woshuas and Aarons both give 52 EXP, so only kill those two. make sure to get the double battle as your last encounter for the extra 52 EXP. Lastly, after passing through the Onion-San room, Shyren is a guaranteed encounter whether or not the kill counter is exhausted, which gives you the last 52 EXP before Undyne.

Sidenotes- If you don't care about max EXP, and just want LOVE 13, you can take off the Don't Pick Ons and the 2nd round of Pick Ons from the Loox, as you only lose out on 42 EXP. you can also take off either the double kill from the Ruins (and the first Froggit), or the double kill from Waterfall, as each would still leave you with 3522 EXP and 3502 EXP, respectively.

Sidenotes, Continued- For your Armor, Temmie Armor is unobtainable in the Genocide route, though it would be the best. Instead, you're left with the Integrity and Perseverance sets. If you tend to get hit a lot of times in a row, use the Cloudy Glasses. If your hits are few and far between, use the Old Tutu. For your Weapon, if you want to optimize damage, use the Ballet Shoes. If you want to optimize survivability, use the Torn Notebook.