Heart of the Swarm Campaign- Zeras: The Crucible (Part 2)

Almost as soon as that attack hits, both enemy broods start sending a constant stream of Zerg across the paths at the Southern side of the map, engaging in a 'tug of war' style offense against one another.  Just to make things more complex, Brakk's brood come in from the East again with a larger attacking force.

Thankfully, Abathur discovers that the energies from Kerrigan's chrysalis can be put to good use- every so often it can spawn a horde of locusts that will move towards and attack the nearest enemy force.  Use this ability immediately to fend off Brakk's attackers- and then keep using it as often as possible to cut holes in the enemy forces.  Even after this attack, there is more than fifteen minutes of time left during which you have to protect Kerrigan's chrysalis, so keep improving and extending your forces and control.  There is a handy supply of minerals and gas to the East of the pool's plateau, so send out a force to secure that and then develop a base there as quickly as you can.

Around the twelve minute remaining mark, Abathur and Izsha will note the presence of an independent and particularly powerful primal Zerg- the Tyrannozor.  This creature and its lair are in the formerly-obscured South end of the map, and slaying it will yield the DNA and bio-mass needed for the added level for Kerrigan.  The creature is massive and powerful, and its lair is on the far side of the clash between the two enemy primals- so be sure that when you send a force after it, that force is either very mighty, or highly expendable.

Tyrannozor aside, this mission winds up pretty easy- only defense is needed.  It's possible to set up nigh-impenetrable defenses at all three ramps leading to the immense spawning pool, and then just build up forces for the inevitable final assault.  Brakk's leaderless brood are eventually defeated, at which point the other brood surrounds you and begins sending in ever-increasing amounts of primal Zerg.  This culminates with a particularly large wave sent around the one minute remaining mark- but this is much too late for them to actually reach and destroy the chrysalis.

Once Kerrigan's change is complete, she is once more the Queen of Blades- in body.