If you really want to powerlevel your chars, wait until you unlock 20-3. Then link the 2x effect and 1.5x xp terminals to the arena using either the straight line colors or the green ones. You need about 80 of them. YOu might need to put a station up to get started.
I farmed the straight line ones on the roads by the forest. There is a rare item and item multiplier right by there and it only takes 15 or so of almost any color to link to those roads right by the forest. You also get mimics there which you should kill the shields for chance for the rare metal shard, which are very useful I heard for the scopes and the physical resist rings.
Once you get to 20-3, there is a gold slime. Kill the other two slimes and then with the person you want to level (wearing EXP trainer from the doll quests, auto trigger and the station bonuses) have them charge up to about 20-25x or so. It should take off 60-70% of the slimes HP. By the time you charge again, it should be full health. If it isn't, then just take off some gun mods. Just make sure the slime is targeting someone other than your dual MG user and stand right next to the slime. Skip everyone else's turn. It should take about 1-2hr at most to level from 20s on MG, to 100. Took me about 70 mins or so.
Do this with all 3 chars and you are now probably set until you need to prep for Neverland.
If you still want to level, do not kill the slime, just exit the arena and do it again. Once you kill the slime, it won't show up again since it will be 20-4.
I just did this last night while watching some TV, looking forward to being much more powerful now. Going to farm Arena 31 and 37, 38? For the rare scrapped parts and some other parts and try to almost fully upgrade my guns for charge accel and charge rate and I should be set for the rest of the game.