Triggered by killing the hatchery on the left of the Sector 1. You can also attack him directly.
Normally triggered by walking on the bridge to the Sector 3.
It can be triggered earlier on rare occasions, likely by aircraft flying to the sector 3, but I'm not sure.
Triggered by walking to the entrance of the Leviathan's Lair. Or dropping a Nuke on his head.
Brutalisk near Leviathan Edit
Triggered by walking in the area to the left of Leviathan's Lair.
He is also triggered if Brain Bug is triggered or his Brain Bug's Lair becomes active. In some difficulties this happens when Leviathan's Lair gets destroyed.
Brain Bug Edit
Attack it to trigger. He's waiting for you on the right of the Brain Bug's Lair. Dropping a nuke on his head or walking into his area works, too.
Three Brutalisks near Brain Bug Edit
They start patrolling when Brain Bug's Lair becomes active. They patrol the high ground of the Brain Bug's Lair. So you just have to aggro them. Watch out when walking on high grounds, as they may aggro on something else, loose sight of you, return to their base and get healed.
Hybrid General Edit
This boss has a lot of triggers, Either by attacking something on the ledge (nuke sometimes works), Blinking on the ledge OR walking far enough into his base from the front.