As soon as you have control of the main character, press
to open the menu. Go to Misc., then system settings, then "claim special rewards" to get some items to help you. Go to your equipement and equip the cobblestone sword, Trodain Bandana, and trodain Togs. You can't use the seeds of skill now, save them for later.
Fight the slimes and move up the mountain, through the cave. Some fights here are optional, and if you explore, you'll find some healing herbs. Fight the smog monsters and continue up the mountain. A cutscene will play. Go back down to where you started. Another cutscene will play and then the achievement will unlock.