The first froggit that gives 10 EXP doesn't count as a kill so it's free EXP, Picking on Looxes allows you to get Extra EXP when you proceed to kill them, so pick on a loox like 3 times before killing it, then you can grind for EXP with "picked on" Looxes until you start genocide, also the last encounter before the "but nobody came" gets triggered needs to be an encounter with 2 Looxes so you have 22 extra EXP, then you'd have LV 7 before killing Toriel, so proceed to kill Toriel, then continue your genocide by going to snowdin, once there, go directly to Glyde, make sure to kill Snowdrake and the royal guard dogs that are before the room where you find Glyde, also, don't activate an encounter with Lesser dog, because it isn't an obligatory kill, but it becomes obligatory if you start his battle, so reload if you ever start a fight with him, Kill glyde, then get out of the room, then get in again and try to kill Glyde again, repeat until you clear out the kill count, then the rest of royal guard dogs will be Free EXP, once you're done with killing Papyrus, go to waterfall and grind on Woshuas and Aarons, do not proceed to kill shyren yet, since you need to clear out the kill count, once you do, then Shyren is free EXP, The last encounter before clearing out the Kill count has to be an Aaron and Woshua encounter, then kill Glad Dummy and then go ahead to fight Undyne the Undying, if you did everything correct until this point, you'd be LV 13 for the point where you fight Undyne the Undying, Then go to hotland, Kill the RG01 and RG02 and Muffet, and then proceed to clear out the kill count on the core, Last encounter before clearing out the Kill count has to be an encounter with Final Froggit, Whimsalot and the third one which i forgot it's name, and boom you'd have the perfect genocide.

if you want to get the highest amount of LV in a neutral run possible, you have to follow all the steps for the Perfect Genocide run up until you get to hotland, you have to abort Genocide by sparing RG01 and RG02, then kill Muffet and clear out the kill count on the CORE.