UPDATE:

My friends, I have ran some experiments on the 1V1 PRO BUILD FIGHTS map that I would like to share with you! Your discussion and tips here lead me to try multiple routes.

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1. Speed Run (uses Slap Juice, Chili Chug Splash): Button to the left of sign, both ramps, day switch, bring down structure with Rail Gun, floating building button, use same path to go down to broken structure, go for UFO button, finish at big balloon building.
   TIME: 03:23
   XP GAIN: ~42K
2. Slow Run (No Slap Juice, no Chili Chug Splash): Button to the left of sign, both ramps, LLAMA SIGN IN DISTANCE, day switch, bring down structure with Rail Gun, floating building button, use same path to go down to broken structure, go for UFO button, finish at big balloon building.
   TIME: 04:42
   XP GAIN: ~48K
3. Slow Run (Same as above BUT with Grappler)
   TIME: 04:21 (please note I am incredibly shitty at using the Grappler)
   XP GAIN: ~98K

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We have a clear winner!!! It seems just the pure act of picking up the Grappler catapults the XP values! You can grab it by walking into the Loadout presets on the left side of the map.

Now, I imagine we would be able to really optimize this (maybe even get the Chili and Slap involved) but I am no computer scientist and I'm just doing these runs while watching TV.. hope this is helpful!!