This one is simple but a little time consuming for the casual player. If you aren't going for the "beat all" missions or the "3 medals" missions then this achievement should be easy.
My recommended route through the levels would be to go straight up from the bottom left starting square and straight over to Satura. There will be four easy levels, a medium level, a hard level, then two more medium levels, an easy level, a hard level, and then Satura.
Also remember for this achievement medals and full completion of Forgotten Forest are not required.
Good Luck!