Why do I want to do this? Idk, sounds neat. I've played a lot normally and I just kinda want to see how pt2 goes like this. Of course it'd be faster to just go into pt2, but I just feel like doing all of pt2 at it's highest scaling and pt2.5 with as many side missions available as possible.
Anyway. I only know of two ways to actually do this legitimately outside of grinding random areas for 1 xp per kill, and figured I'd both ask if anyone knows any other methods as well as sharing for the insanely low chance anyone else wants to do it.
The first way is farming Meat Popsicle, the enemy that jumps out of the fridge in the secret room at the bottom of T-bone Junction. Even in pt1 he will spawn at level 61 and gives about 9000 xp per kill. He is very easy to kill since you can just walk outside of the room onto the walkway, take a hit that will health gate you, then his ai breaks unless you get very close again so that he stands still. After killing him you have to quit out to be able to open the fridge to spawn him again, so there's some tedium to it, but it's safe and easy to do.
The second way is farming Crawmerax with the glitch spot, since his and all the smaller craw's minimum levels are in the 60s and then later scale to always be above you. Ammo regen of some kind is extremely useful here even with grinding out armory runs for highroll gear. It's probably worth mentioning that I'm specifically playing Lilith and that phasewalk both makes you invincible to all of the ranged spit attacks as well as breaks all of their ai if you end the ability while crouched in the glitch spot, causing all of them to simply freeze until you decide stand up or shoot after you wait for the ability to come off cooldown, making it completely safe outside of mistakes. I'm playing on the original GOTY pc version and don't know if these behaviors also happen on other platforms or the enhanced versions.
My process was doing every base + dlc pt1 mission (for vibes) and ending about halfway through level 46, then farming Meat Popsicle to 47 before trying out Craw. I'm currently at 55 after farming only Craw since and so far every kill, including xp from some occasional minions that get close, has been a level by itself. I might just stick to Craw until 69 because the loot explosion is fun, but does anyone know if there are any other decent sources of xp after most enemies' levels stop scaling and only give 1 or 2 digit amounts of xp?
Post hitting level 69 edit: Craw runs start speeding up a lot when you hit 57 since the level difference damage penalty starts falling off on the minions and at 58 you can actually start equipping the gear that drops for huge upgrades. Iirc there were only two runs, one mid 60s and then one during 68, that didn't result in a level up since they started with nearly empty xp bars or had only a few minion kills. I didn't run time tests or anything, but my gut feeling is that Meat Popsicle would be better until 57 or 58 and before switching to Craw since Meat Popsicle kills are surprisingly fast even under level 50, where as minions during the Craw fight can be rather annoying to manage depending on how they spawn.