So to utilise this method you need to have the mission bond episodes for Tsugumi (Yuito) or Kyoka (Kasane) available, so if you've finished the game or completed them you'll need to load up a new ex game and have to get to the point of which these are unlocked around phase 11 to farm xp (very good for end/postgame/achievement hunting). You'll go out with your companion to Hieno Mountain for their respective quest.

Instead of going to the moutainside head to the trailhead and start clearing out the others. In the bond missions the enemies levels are scaled to match yours and that's an important note to come back to later. When you go out, have some plug-ins that are going to help with the method and xp farming such as the bonus xp, specialise in crush, SAS recovery plugins, brain drive accelerator.

Your best friend for xp is the flying exploding others (Session Pound), they give you the most xp, you want to paralysis them so you can stack up your xp multiplier. Then brain crush the monsters when it's maxed out. I found psychokinesis, duplication, and hypervelocity and of course electrokinesis super beneficial to stop them from exploding so I could paralyse, grind, and crush them to get the most out of xp.

Make your way through the trailhead and head into the moutainside repeating this process and whenever a big stack of session pounds are around use brain field to up the cap on the xp multiplier and keep on crushing those session pounds and any other enemies to get the most out of the xp and keep your SAS gauges going. Once you've gone through all of the other spawn areas DO NOT GO TO THE OBJECTIVE, travel to a different area other than your hideout and then head back to the trailhead to rinse and repeat. Once done just finish the mission :)

When you level up the others levels scale up (BUT ONLY IN THESE AREAS IN THESE MISSIONS) to match yours giving you even more xp. I've just reached lvl99 from it and a singular session pound was giving me around 100k and the bigger, less reoccuring enemies will give you even more the more you level up around 300-600k roughly.

In the video I made and linked I stop at the big clearing once I fully max out my characters but you can go further all the way until the objective doing the same thing. There’s a tough less common monster (I’ve forgotten the name) there that if you do crush it in a brain field will give you 600k+ xp around level 90+.

Enjoy getting maxed out characters!