Crystalisks are large, three-legged creatures that are mostly comprised of rock and crystals, with the latter primarily appearing on their legs and back. In fact, those very same crystals are the creature's weakpoint, as their bodies are very resilient and can reflect bullets. When fighting them, you will want to aim at the crystals on their legs, as each of them actually makes up one-third of their total health, so destroying all three will kill the beast. Just note that after two of their legs are gone, they will actively try to hide their third leg by spinning away from you.
The Crystalisk has four attacks, two of which are executed in melee range and the other two as ranged attacks. For melee ones, it will lift up one of its legs and stomp the ground with it, creating a small explosion in the process. The other melee attack is it pounding the ground repeatedly with its body. One of its ranged attacks include launching missile-like projectiles. The other one is a large crystal that sticks to the ground and will detonate after a few seconds.