Realms of Arkania: Blade of Destiny

Note that the achievement requires you to remove all of your party's armor, not their weapons. So you won't be completely defenseless in a fight. I also recommend that you do this as early as possible, against small groups of enemies, or large groups that are weak, like wolves.

When in-game, press to open your inventory. Then remove all of your armor from your character by scrolling down to each item and pressing . You'll need to tab over with to take the armor from the other companions too, as the achievement requires an armor rate of 0 for the entire party. Once again, weapons are excluded.

Once you've done that, you'll need to enter a battle and win. An easy way to get into battle is to fast travel, and hopefully, you'll find an enemy on the road. Some enemies are obviously harder to kill than others, so I would avoid any directly dangerous locations like bandit dens or caves. Try to find wolves instead, as mentioned above.

Then, all you need to do is actually win the battle. The achievement will pop once it's over.