You should earn this achievement during the final mission. However there appears to be a glitch where if you run directly towards the middle without going towards her spawn location you can miss this boss and be unable to earn this achievement.
The Opha Superior is a mini boss near the end of the final mission
This enemy is found in Elesser and spawns TONS OF GUARDIANS to help her fight. NOTE: She has an extremely strong shield so get out your shock weapons!
I found that the guardians were the only difficult part of this fight since there are SOOO MANY!
I solo'd this at level 28 and wasn't a problem at all.
Overall this achievement is fairly easy.