In order to complete this achievement YOU MUST COMPLETE Barbarous 7 times. You can do this SOLO or in COOP. However, I found SOLO to be easier with the info below. This is a complete list of tips with the BOLD ones not being listed at time of writing.Much faster than the video solution.
A full list of tips to make this much faster.
Best to use Jupitar. Stunning the few you need to works best.
In the second battle, use the mortars to take out some while you are playing. However, it is better to build up your Focus.AFter you get to the town, there will be two enemies. LET THEM JUMP DOWN. And kill them WITHOUT GOING UP. After they are dead, climb up.
Get on the arrow machine and wait. The stone (colored in yellow will fall by itself). Aim just above a little post and wait. Enemies will come up. If you aimed high enough, you can kill them all with the arrows. DON"T WASTE SHOTS!!!
Missed a few? Get off and kill those you didn't hit. Focus stun helps here. Kill them and get back on. A duel sword enemy climbs the ladder. Kill him.
Jump down. Kill the enemies through the fence with the arrow machine. Open the game. Get back on the arrow machine and wait. Kill the harder enemies with the arrow machine. Run in. Step on left then right, climb up and tip over right then left urn.
NOW! here's the easy part. Immediately RUN RUN RUN back to the arrow machine on the platform. RUN.
The enemies will follow you. and you can kill them with the arrow machine. EVEN THE LEADERS. If some make it through, JUpiter will help here again.