Double Dragon

Idiots guide for Double Dragon in 6.0 (Healing Class Edition)

1. Agro [npc=45992] (but don't burn her down)
2. She casts [spell=86788] twice. Soak it up and top up your health.
3. [npc=45993] will then cast [spell=86408].
4. Stand in the shadow-pool caused by [spell=86408]. This will take you into the shadow realm.
5. Target [npc=49864]s as they appear and kill them. You need 6 for the achievement.
6. The achievement should pop when you kill the sixth [npc=49864] if you have downed the boss on a previous run.

There also seems to be only one fiend present for each phase transition, meaning you'll need to enter the twilight realm six times to get enough for the achievement.

This is untrue.

They spawn at intervals of roughly 10-15 seconds.

This achievement is semi RNG, depending on how many people you send into the twilight realm.
It is possible to get all six in about a minute in the realm. The fiend spawn in random locations... and run what seems to be 25-30 yards before despawning. The trick is to spot them asap, slow them, then kill them. Problem being the random locations. They can spawn anywhere. etc, center of the room, near either of the bosses starting points. The room is 180y x 60y. In theory for 10 man, if you have 3 people stationed in the center line longways, preferably ranged slowers on the top stairs & a good rogue/dk stationed in the center, 1 center and 2 on the top of the steps spaming the macro

/tar twilight fiend

you shouldn't have a problem meeting the criteria in 2 Dazziling destruction phases.
Some reason, people get insta jibbed in the DEEP BREATH, might be a timing thing of running in and out quickly, needs more testing.

You will accumulate 12 stacks by the time 3 fiends spawn. Cloak, bubble, non specific despell ablities are your best friend in increasing your kill counter. Also, avoid the "real" realm AoE abilites from the boss at all costs. 10+% per stack shadow damage + unavoidable AoE + Boss abilites + %^&*ing orbs = oh @#$%.

In short, As long as you have enough people covering most of the ground, killing 6 fiends shouldn't be a problem.

A reference to the popular video game Double Dragon.