This achievement is obtained after defeating Saranda at the end of Valette Castle.
Luckily when fighting Saranda you wont have to worry about any black orbs, but in return she has many damaging area spells that can cause massive damage if everyone is caught in them for the full effect. Just like any other boss fight, keep your HP high and keep pummeling her, if you are running low on health, don't be afraid to back off and heal manually yourself.
Just like prior boss battles after damaging Saranda enough she will kneel down and you can now attack the chain. Do this 3-4 times (depending on if you damage the chain enough) and after the final time Saranda will die and the chain will break in one hit.