Water Ink Ninja (Washi) along with his fellow Ink Ninjas are hands down the best farm team due to the raw, hard-hitting power of his ultimate. When he lands his ultimate with an enemy marked, the bonus to critical damage can wipe out enemies with low HP pools.
I suggest Adept/Precise for a few reasons. Yeah, Washi's crit damage needs to be high, but you aren't always going to kill 6-1's scorpion in one hit. It'll usually take two, so getting the cooldown on Ink Spirit Calling as low as possible is key to a fast and effective run.
Accuracy is vital, as Washi relies on his ability to mark enemies to guarantee crit hits with his ult, so I equip him with precision. Gear should all be focused toward Attack % and Crit Damage %.
Farm 6-1 exclusively. This is going to give you the most XP for your money. Use Washi's passive, which will give the group 33% additional attack. For the most effective farm, manually click on the scorpion to focus your units. You don't want Ink Spirit Calling wasted on trash.
When he is fully geared and tutored, you will be running 6-1 in 12 to 14 seconds. With decent gear and a few tutors, you'll run between 15 to 30 seconds. That is fast.