**This achievement is missable**
You will need to invite Pen to a sparring-duel and win.
Warning:
*** Spoiler - click to reveal ***Pen will leave Sandrock eventually, so you must complete this achievement before then. If you do not complete the achievement before then, you will need to start at new save and try again.
Tips and tricks:
*Note that below is not necessary, but will make the sparring duel a bit easier*
Recommended level: 30+
Recommended weapon: Daggers. Other weapons may work too, but since they are very slow, it is noticably harder, since Pen's punches are super strong and you need to dodge them.
Recommended armour: Damage reflection. You can get a whole set from Vivi's tailor-store already from the beginning of the game. This set should suffice.
Recommended Skillpoints: All stars in daggers, some extra crit damage and some +BREAK, and all stamina skills.
Notes about the combat:
In the first half of the combat, Pen will use "light" jabs, that hit fast. You will need to try to dodge those as best you can, while trying to get his HP down as quickly as possible.
When you have taken about half his HP, he will start using charge-attacks that are way easier to dodge. Make sure to dodge every one of those, because they will take a lot of your HP.
If you have not lost HP during the battle and are running low on time, let Pen hit you with 1-2 attacks if you are wearing the Damage reflection armour, since he will deal massive damage to himself.