SWORD ART ONLINE: FATAL BULLET

Xp specific method, lvl 75+ when the leveling truly starts slowing down, and you have enough CP: (make sure you're on extreme mode, so you need to have completed a playthrough, and you also should have Bazalt Joe in friends list, awarding dual-wield)

Go to SBC Glocken, go to the shady item merchant, buy a Lupus 3 assault rifle for 1-2 million credits

Now go home. Go to your customization console next to your arfa-sys. Save your current customization if you want. Start/load a new one. You want to load up your STR/DEX for dual-wielding your new Lupus 3. I think for me it was 105 STR and 125 DEX (note that these will have to go way higher if you enhance your Lupus 3 or find a better one).

Distribute your remaining CP probably into VIT/AGI/INT depending on what skills you use. You might need more CP from leveling up. I personally use Power Boost, Armor Boost, and the Armor Break Bullet

Go to equipment in your status menu and finally hit the Y button or w/e to dual-wield the Lupus 3. It should give an exact duplicate in your off hand.

Now go to Remnant Wasteland or Solitary Sands or wherever, and generally your DPS will be way higher, massacring the enemies. (You should be focusing on weak spots also when you can)

(Note: after about an hour I could roll out with almost 500000 bounty also, mainly from enemy killcount and materials harvested)

It gets kind of grindy at 70+, whether on Normal where you run around doing an enemy massacre, or on Extreme just trying to kill stuff anywhere. (Note that on Extreme the enemies aren't necessarily smarter. The pvp AI players might throw more grenades/use more skills but they don't improve their tactics. The npc enemies simply get a level boost on Extreme, about +60-70 levels, give or take, to whatever level they are on Normal. For example, a level 27 Scorpion in Solitary Sands on Normal will be a level 88 Scorpion on Extreme. )

If you want to find better weapons to help your leveling, this is a double-edged sword. Bosses drop specific things, but there aren't as many bosses as there are types of weapons. The Subway B final boss (similar to the one Sinon kills in the Anime for her Hecate) will always drop a Tiamat sniper rifle. Its level 120 on Extreme and drops a rank 8 Tiamat. But the best place to find rank 8+ weapons is actually the Gold level BoB Simulation from the Governor's Lobby. They are random uncommon or rare drops off each AI player you beat (that's right the BoB Simulation is only against AI players so you don't have to put up with human B.S. yay!). The problem with this is that Gold Level BoB is level 160...so your best strategy for this is to use Armor Break skills, combined with an automatic weapon, combined with never stop moving/dodging...unless you're close to level 160 then you might be able to take some of the punishment...your biggest concern will probably be the AI throwing grenades because those do tons of damage.

Another problem with finding better ranked weapons is that you want to use Transmute on them anyway to improve their stats/bonuses at the same time by grinding out tons of the same weapon and combining them to increase the percentages, because weapon rank isn't going to quite cut it on Extreme, or even the tougher Normal areas (looks like Demon Cave in Solitary Sands is the toughest).