I Love Snakes  

During the Unveiling the Mystery mission, two bonus boss mobs spawn after you've killed the 4 portal guards and finished talking to Calvin Tresc. One of them is Szalvinzir which spawns in the arena. Szalvinzir has 5 phases:

1 Szalvinzir, the Arena Champion
2 Szalvinzir the Great
3 Szalvinzir the Mighty
4 Szalvinzir the Revealed
5 Stone Memory of Szalvinzir

During phase 1, he continually spawns a large number of level 75 "a hissing serpent". The achievement wants you to keep 100% those hissing serpents alive until phase 4 begins, which is next to impossible because they have such low HP that even the cleric merc riposts them to death.

The hissing serpents are mezzable, but trying to keep them all contained wasn't working. So what I did is near the start of phase 1, I harm touched Szalvinzir for 2 million which killed him before any hissing serpents spawned. When we reached phase 4, our group was awarded the "I Love Snakes" achievement.

All event mobs are leashed within the arena boundary. If anything goes wrong, you can reset the event by leaving the arena through the door. Drop aggro if possible.