So if you still went for Max Exp in the Core you will need to Kill R1 and R2 (if you wanna stay on genocide) Kill muffet and Kill 36 Knight Knight. Then find a Triple encounter. After that go to Warriors Path and kill the 7 monsters in the path. You will have 13,376 Exp (Level 16). Before Reaching Mettaton Neo. Theres no known way to get to Level 18 without editing or hacking files. Or playing the 1st version of undertale and abuse the Loox exp.
So if you still went for Max Exp in the Core you will need to Kill R1 and R2 (if you wanna stay on genocide) Kill muffet and Kill 36 Knight Knight. Then find a Triple encounter. After that go to Warriors Path and kill the 7 monsters in the path. You will have 13,376 Exp (Level 16). Before Reaching Mettaton Neo. Theres no known way to get to Level 18 without editing or hacking files. Or playing the 1st version of undertale and abuse the Loox exp.