Grindstone

At least so far I have gotten the maximum chain you can get is 48 (7x7 grid - yourself). Even with that, there are most often obstacles in the way too, but this is how I got the achievement.

1. Go to the inn, in the bottom floor you will find Helgä''s Slöp Höuse Slöp Bucket.
2. Go to the second from the left button. It's a picture of a bucket. Buy the snack Eat Your Greens. It costs 6000 slop, you get that by killing enemies. Add it to the lunchbox to take it with you on the next level (also each snack is a single-use item)
3. Go to Level 15 on the mission board, use the snack and roll through at least 45 of the green fellows. Reason for level 15 is because it's the first level with access to that many enemies.

Anyhow, congratulations on the achievement!