This mechanic begins when Brute Bomber starts casting Barbarous Barrage. Shortly after the cast ends, 6 knock-back towers will appear, shortly after that a center knock-back tower will also appear.
The two middle towers will go off first, and each need 4 people stacked to them. The corner towers will go off next, and need 2 people stacked. The central tower that appears slightly later, will need everyone stacked on it and goes off last.
The 6 towers that appear first will be in two lines of 3, positioned either North and South or East and West.
At the same time that the knock-back towers are resolving, Brute Bomber will teleport to a corner and begin to cast "Murderous Mist". When the last knock-back tower goes off, make sure you are positioned so that you get knocked-back to the same corner that the boss is in.