Leveling in Nightfall..

How do I close the gap between 6 and 12 for the Jokanur Diggings mission?

The quest you've got says to either level upto 12, or gain the rank of first spear.

What i do is go into zehlon reach, accept the bounties, and go round killing stuff. Also there are a few quests to find in zehlon reach.

Aboslutely the fastest way to progress at that point is to get to 1st spear. Pay attention to where the bounties are given & where the critters spawn.

For example: When doing the quest with all the harpies & their nests, the shrine with the bounty for harpies is right there, so detour around the harpies & get the bounty. It's not out of the way.

Or try going west from Jokanur Diggings outpost and grab the skale bounty in Fahranur. There is a large group of low-level skale at the gate to the closed-off city that is pretty easy to just wipe clean and do again after /resign.
Will get you to first spear in no time.

Every blessing, every time. No excuses. You level up crazy fast. And while you're at that use XP scrolls at the same time.

You don't even need to do this. If you do all the quests you'll meet both of those requirements without much fuss.