This is a guide to farm EXP. I only found this reliable source. (Been testing myself)

Step 1: Get Sagestone [remnant2.wiki.fextralife.com]. You get it in Yaesha. Drops on The lament and The Twisted Chantry. You can also lvl up the trait Scholar to lvl 10 for an extra 15% to a total of 25% extra EXP.
Step 2: Reroll Yaesha until you get The Chimeny
Step 3: Reach the event where you have to have to go down while fighting hordes of enemies. Do not finish the event, instead, stay on the bottom part and kill as much as you want. The event will never end unless you destroy the last roots.
Enemies spawn ammo and scrap so you can get easy EXP for your archetypes and some scrap. Lvl 9 to lvl 10 can be farmed in 10-15 mins depending on how fast you kill the enemies. Better if you have the Enigma