1. use loox "pick on" 3 times everytime in your only-loox kills in the ruins

2. dash through snowdin quickly without killing anyone, reload save file if you encounter lesser dog (since that's lost EXP) and go to the room with the mysterious door and walk around untill glyde appears and kill him then exit then re-enter the room repeat until glyde is ded 16-ish times

3. grind aarons (and woshuas too if they give the same EXP i guess) before the 1st undyne encounter since shyren and mad dummy counts as kills (if you did this perfectly (including that previously mentioned free froggit and the bonus step later) then you'll be LV 13 when you reach undying undyne)

4. in hotland for absolute maximum possible without triggering the mettaton NEO auto LV 19 kill then skip all hotland enemies and spare the royal guards (since they give less EXP than knight knights)

5. in the core kill only knight knights since they give the most possible EXP

bonus step: for the last kills set yourself up with specific matchups to get a bit more EXP: ruins - 2 looxes when you're at 19 kills (you have to keep track) waterfall - an aaron and woshua the core - astigmatism, whimsalot and final froggit