When Anomalus shrinks and becomes invincible, all of the active rifts begin spamming an arcane Chain Lightning type attack that deals a moderately-low amount of damage (around 300-500 a tick). While it doesn't seem like a lot of damage, each rift does this damage to the entire party every two seconds. It adds up.
Now, while it looks as though the rifts zap each party member individually this is not the case. The rift lightning is in fact a chain-effect that targets the party member closest to the rift before arcing to another party member in the chain. This means that if one party member is closer to the rifts than the rest of the party, and there is a large enough gap between that member and the rest of the party, then the lightning will not arc to the rest of the group.
NOTE: The rift lightning will not chain to or from pets.
The wraiths are social with their rifts. If the rifts can zap you, and they can, then the wraiths will aggro to you no matter how far away you are.
The requirement for this achievement is to kill Anomalus without killing any of the rifts he opens. Where normally you DPS in a simply priority setup (Rifts, then wraiths, then the boss) the achievement makes things a bit different. In order to defeat Anomalus you will need to reposition the party every 25% of the boss's health to a new choke point, position all party members correctly in order to minimize all damage from the rifts' Arcane Lightning attack, and kill a lot of arcane wraiths.
Any group makeup will work, although there are ways to make your life easier.
- Ranged AoE will make killing the massed wraiths trivial. Mages, Warlocks, and Hunters are good for this.
- An AoE tank will make tanking the wraiths possible. Without this your healer's life will become a living hell unless the ranged DPS can kill the wraiths before they deal much damage to the healer.
The key to this achievement is properly positioning your group members during the invincibility stages so that only the tank is getting zapped by the rifts and has the opportunity to intercept the incoming waves of wraiths. Proper positioning for the group is behind a choke point with the tank between the rest of the group and all spawned rifts. The tank must be the closest party member to the rifts so that he gets zapped rather than the rest of the group, and he must be far enough away that the attack does not chain into the rest of the group.
At 75%, 50%, and 25% HP when Anomalus shrinks and becomes invincible the party will have to deal with the lightning attack channeled by all active rifts as well as all wraiths that spawn (regardless of their distance from the rifts. Rifts are social with wraiths and the wraiths will aggro to the party when the rifts zap them). The point of a choke point is to funnel all spawned wraiths into a common area so that they can be quickly killed with AoE as they approach.
The strat is simple: Tank Anomalus until he shrinks. When he does the entire party turns and runs making sure that the tank is the last one to leave. When the party gets behind a good choke point the tank eats the lightning, the healer focuses on the tank, the ranged DPS AoE any and all wraiths that come to play, and the melee chill until the shrink phase is over. When the shrink phase ends the tank picks up the boss and the strat is repeated.
After the third channeling phase is over, the fight is relatively simple. Just continue the fight as before, being cautious of any adds that have spawned and are attacking the tank. If the tank dies, Anomalus can easily be kited back towards the front entrance of the instance while ranged dps kill him, ignoring the adds. Keep in mind that a path will have to be pre-cleared before you attempt kiting, as Anomalus can cause nearby mobs to aggro, wiping your group!
This achievement is perfectly possible with a decently geared group in 80 blues and epics from heroics. Since the majority of damage dealt is Arcane, some Arcane Resistance gear will help to significantly lower the healing load required of your healer, however it is not mandatory.
Arcane Resistance is far from mandatory. Properly positioning the group will make life far easier than any amount of Arcane Resistance. Combined with a well timed cooldown, positioning and a good series of choke points will leave you wondering how you ever thought this achievement was hard.