This achievement is obtained after defeating Veros in the Lunar Sanctuary.
Prior to fighting Veros you will have to fight Leonid one more time, he is exactly the same as before, besides that he likes to spam VIIth Violation a lot more often. Use the same tactics to take him down (Also if you defeat him within 2 minutes you will get a situation bonus). After defeating him you will then have to fight Veros.
As the fight begins Veros will be surrounded by these sort of shield things, he will spin them around and depending which lands in front of him it will be a different attack, most of these attacks are unavoidable and lucky if you don't get hit, mass elemental spells and some cause some status effects. You must destroy all of these "shields" try and concentrate on one at a time and set your tactics to Focus, also be careful when he spins them you can get hit by it.
After defeating the "shields" two black orbs will spawn on the sides of Veros, you must now destroy these, concentrate on them one at a time and they will go down quickly. Luckily during this phase his attacks are a little more avoidable but still cause big damage.
After destroying the orbs you will now finally be able to attack Veros, make a final onslaught and take him down as soon as possible, at this point he has very low hp left so make sure and take him out before he lets off any super strong attacks.