Efficient killers, acid shells possess a resistance to energy fields and weapons. Their powerful mutant armor provides them with mobile fortresses into which they withdraw whenever they're sufficiently injured. Once inside their armor, shells spin a radius of death, spraying acid on any foes in range.
Acid Shells will spit acid that will deal energy damage. They are highly resistant to Energy Attacks when outside their shell, as seen by their glowing yellow sheen.At half-health, they will retreat into their shell for a period of time, emitting a deadly cloud of poison in all around itself during that period, poisoning all nearby Heroes. If you happen to get poisoned, you can use Meditron's Syndrome Shift or Tork's Sporegenesis to cure the effect, but be very careful when using Tork as the Acid Shell has the same genesis type as Tork. At this point, they are extremely vulnerable to Energy attacks, but are immune to physical damage. If left alone long enough, it will exit this state, and become resistant to Energy Attacks again.
Krel is an excellent choice to counter Acid Shells with. He can run circles around the shell, hitting them with Tribolts and Twinblazes. Twinblaze will deal energy vulnerability, especially helpful when in Co-op with Tempests. Once the Acid Shell has entered its second phase, players can use Krel's Supernova and any energy based Variant Abilities he has. Should the Acid Shell survive that and return to its mobile phase a few Twinblazes should finish it off.
- Acid Shell's behavior of retreating to its shell is similar to real world turtles. They even resemble turtles when they retreat into their shell.
- During the Beta, acid shells had a slightly different appearance and appeared in earlier levels.
- Acid Shells can be seen in the Enemy Spotlight, featured on Scaldron, also in the Cryos Co-op Walkthrough, various Beta Livestreams, and a few other videos from Maxis.