Starcraft 2: Heart of the Swarm brutal campaign walkthrough

KALDIR MISSION 3: ENEMY WITHIN

Main Objective: Gain Biomass [0/100], Find a Safe Place, Kill the Protoss [128 remaining], Niadra Must Survive, Destroy the Warp Drive, Escape to the Stasis Quadrant
Bonus Objective: Infest the Giant Ursadon

ACHIEVEMENTS

Enemy Within: Complete the "Enemy Within" mission in the Heart of the Swarm campaign.
Biomass Effect: Collect 450 biomass in the "Enemy Within" mission.
Failure to Launch: Don't let an Escape Pod reach less than 20 seconds in the "Enemy Within" mission on Normal difficulty.

If this is your first play through, don't bother trying to infest the Zealot. You can only infest animals in this mission.

Once the sentry has passed and come to a stop, it is safe to move to the next area.

Use the Ursadon's leap attack to deal with the attacking Protoss units.

Keep spawning zerglings as soon as they die. They don't cost anything so you can essentially spam it in order to reduce downtime.

Regardless of what's thrown at you, the ability to spawn additional zerglings for free, make this mission extremely easy. If you chose Raptor Zerglings, this gives you even more of an unfair advantage.

The Protoss that awaken in this area will attempt to destroy the biomass. Take care of them, while you absorb the biomass around the room, while continually spawning zerglings as they die.

Rinse and repeat with this room as well. Bear in mind with your current form, you can spawn roaches as well. Once you're done absorbing the biomass in this room, you will go through another transformation.

As soon as you destroy the door to this room, send in your units and go straight for the Giant Ursadon. Wait too long and it will die. Absorbing all of the biomass in this room will award you the Biomass Effect achievement.

Supplement your zerglings with a few hydralisks and roaches. The extra dps will help you with the Failure to Launch achievement. After you destroy the first Escape Pod, destroy the two the left and then head directly to the right, ignoring the protoss on the platform.

Add more hydralisks to your composition for additional dps to deal with the stronger protoss units on the platform guarding the last escape pod.