I’ve never done it myself but this is my rough understanding of how it works.
You have a party member cast a ground-magic skill like Cure16 or Frost Cloud and you capture it as a rogue. When you reuse the skill again, all the damage from the skill is considered a physical attack, meaning that it can crit and get buffed by Running shot.
So you capture a cure16 skill, when you recast it, it gets converted into a 280% damage cast of the original skill operating off of your Physical Attack, and with sneak hit at level 5 you’ll crit at 70% from behind. This can lead to some insane amounts of burst, more than enough to do 1/6th the health of boss like dullahan (takes 2x damage from cure16) almost instantly.
The biggest problem is that once you cast it, you’re pretty much dry on dps for 3 minutes, so if you miss it or the boss moves out of it, you’re ■■■■■■. Also if any one else reversi’s it, recaptures it, or in anyway nullifies it, you’re pretty much guaranteed no cube.