1. Pick Quickling
2. Pick Nomad
3. Pick Dry Marsh
4. Stand still for 30+ minutes until the final boss arrives. Mobs will run into you and give you XP.
5. When the final boss spawns, run to one of the far corners and let mobs run into you. Wait for the boss to get close, then run to the opposite corner. Repeat until level 77.

This process took me just over an hour of in-game time (a bit more than that in real time). I hit level 70 around the 30 minute mark, and got another level every 4-5 minutes after that.

1. STUDY (get this to level 10 ASAP for maximum XP, try to get this by 5-6 minutes in)
2. Stamina Potion/Hourglass (so your weapon skill activates quicker and lasts longer)
3. Defensive Passives (HP Regen, Defense, Evasion, etc.)
4. Death Sting (this will kill non-boss mobs instantly)
5. Any other passives

IMPORTANT: AVOID ALL ACTIVE SKILLS since they might kill the final boss (we don't want that)

By following this strategy, your weapon skill will kill any enemy that comes close and automatically scale up in damage as you level. Maxing out Study ensures you hit level 77 as quickly as possible (the earlier you do this, the better). Maxing out Stamina Potion and Hourglass make sure your weapon skill is nearly always active. By filling in the gaps with defensive passives ,HP regen, and death sting, you should never die and instantly kill every enemy that comes near you. Even bosses will be melted in seconds thanks to your weapon skill damage.