Grinding is fine. That's the beauty of this game. You have more freedom than most other MMOs as far as progression goes.
Nogg Road, Desert Wolves, Port Malaya Hospital, and Splendide / Manuk area are where I leveled as a Mechanic during my downtime and I imagine it's not too different for Genes since they both use the same kind of ammunition. Monsters to focus are Nightmare Terror, Diabolic, Cornus, and everything in Bangungot Hospital 1f and everything in Manuk Field 03. My favorite one of all those were Cornus, because with cursed water+dark cannonball combo I'd kill them in one shot and they are passive so it was super safe aside from being ambushed by a damn Naga. Nothing a Fly Wing couldn't fix, though. Their drops were good money too.