Quick Boss Guide: Echo of Andariel
Preparation
Summon: Andariel will appear (materialize) in the center of the room where the summoning stone was.
Andariel will begin phase at various health breakpoints where Andariel disappears and is untargetable.
When Andariel disappears, three 3 effigies (Effigy Of Anguish) will appear in each 'corner' of the circular room, creating a triangle.
Andariel will return once and only when all Effigies are destroyed.
Shortly after the effigies appear, a slow rotating thick red line will appear which rotates around the room until expiring, dealing large damage to anything within the red area.
The damage from the red line gives an exponential debuff stack.
This will continue to appear and rotate intermittently (?) as long as the effigies are alive.
After the second (2nd) Phase/Intermission, when Andariel re-appears, a thin line of fire appears dissecting the room into halves. This fire line will rotate around the room and should be avoided.
Each time you walk through it (take damage from it), an exponential debuff stack is acquired. 
FIRE LINE GIVES A STACK
Andariel will use a knockback frontal (can affect multiple players if close enough).