The level cap with the Title Update is 275, so you basically have to go all the way to the top.
If you're out grinding several levels at once, I can tell you that it costs about 4,000,000 experience to go from 274 to 275, where at level 140, it costs about 1,000,000 experience to go from 140 to 141. So, not too bad.
This is possible in the base game using a beefed up sniper rifle (AMR Deathwind or the MK2) and rotating Explosive Shot skills 1,2,3, and maybe 4. Go to the Forgotten Woods and from the main Field entrance head up the slope to the right and along the edge of the map towards the Flugel Bow entrance. You should encounter 2-3 Player A.I. groups, which die incredibly fast to the sniper rifle Explosive shot skills. Then just reload the fast travel point in the Field, and repeat. (Might take some practice to get more accurate. I found it worked better to shoot the ground close the AI's feet)
If you've purchased the Dissonance of the Nexus DLC, this becomes insanely easy. Once you Start a New Game, choosing the DotN DLC from the main menu, go talk to Argo the Informant at her spot in the Urban District in SBC Glocken and purchase all her new information as soon as possible!!!
I didn't talk to Argo until the very end, and I'm still kicking myself. She sells 5 new things:
1. Memory Chip Replacement (10,000,000 credits - one time)
-allows you to equip your A.I. non-ARFAsys friends (e.g. Kirito, Asuna, etc.) with better gear
2. Accessory Drop Boost (1,000,000 credits - reusable, 100 minute duration)
-turns all accessory drops into epic(green) quality
3. Barrel Drop Boost (1,000,000 credits - reusable, 100 minute duration)
-not sure, says that all weapon drops are at max power, but this doesn't seem to apply to quality, maybe lower level areas will drop rank 11 weapons?
4. Memory Chip Averaging (1,000,000 credits - reusable, 100 minute duration)
-not sure what this does, I haven't changed the memory chip/items on any AI players but it might increase the item bonus stat %
5. Hidden Event (50,000,000 credits - one time)
-allows access to the completion of the Emblem Earrings event for the School Uniform costume set
With those new Argo sold Memory Chip Replacement options, you should be able to equip the A.I. players with EXP bonus weapons and accessories, dramatically increasing your EXP gain in general. The "Change Memory Chip" section will appear in the Friend List tab in your pause menu after purchasing the info from Argo.
Keep in mind that the base game also had an update that added quests allowing you to improve your home. Lisbeth's home improvement quest asks you to obtain 5 Plated Armor and 5 Plated Mantle from the Samurai Brothers-XXX at the end of the Demon Cave. When the quest is done she installs an Accessory Enhancement machine which allows you to improve stats on Accessories. It doesn't let you pick and choose your stats though, so use Argo's new Drop Boost to farm epic accessories until you get something you want to upgrade.
On top of that, the new open FIELD area White Frontier in the Dissonance of the Nexus DLC has base game bosses located throughout the area. What is different is that if you kill the boss, then walk 100 meters in any direction and come back, the boss will respawn, allowing you to farm the boss repeatedly without fast travelling.
The White Frontier monsters also give significantly more EXP than base game monsters, around twice as much without EXP bonuses. In addition, the Scorpion and automaton trash monsters are no longer bullet sponges, and die rather quickly even when first trying out the area.
In the beginning I got lucky with an epic Peacock 4+ from one of the drones and I dual wielded that, but I eventually started farming Waste Gang, who is located north/northwest of the Field entrance fast travel point. Waste Gang is the boss who can drop a Lupus 4+ rank 11 assault rifle, which is an excellent optical-based damage for grinding EXP.
Near 275 I switched to the Gatling Gun. I use an epic World's End++ which dropped from one of the S-Type Sniper Buttlerflys. I beef up all of my weapons at Lisbeth's Workshop, buying Transformation Chips from the Lobby Medal Vendor. The Party Hero Quests #16-18 are great if you need to grab some more medals in a hurry, as the give 300 each after completion in just a couple minutes.
Let me know if I can add anything to make this guide better.