Keep Rollin' Rollin' Rollin'

If you're having trouble doing this post 6.0.2 like I was, I can confirm you can let the stacks fall off and then continue hitting him with barrels and it still counts. I did 6 stacks, then 3, 4, and 7 and it gave me the achievement.

Guys, forget about taking off gear, res sickness and all that nonsense. Just buy [item=88492] from [npc=59822], which transforms you into hozen. If you hit the boss with a barrel in that form it will not damage him at all, but give him debuff. Get 5 stacks, let it wear off, repeat 3 more time, kill the boss, get the achievement.

First time i tried to solo this achievement, i just kited him through barrels, he was getting stacks, after 40+ barrels he died, but no achieve.

Today tried again, this time i hopped on barrels and rolled them into him, after around 24 barrels i killed him and got the achieve, so my guess is that unmanned barrels, though they give stacks, don't count. Also when he died he was at 8 stacks (the stacks fell off at one point), so stacks doesn't seem to be required. Hope this helps :)