Darklings are drawn to possess vicious predators, making the Grenchler a prime candidate. Despite the added toughness gained from possession, the Dark Grenchler retains its one weakness. Remove the back shell with explosive blasts to expose its pain center, then attack the pain center to quickly dispatch the creature.
Grenchlers and Dark Grenchlers are very similar to the Sheegoth creature that Samus fought on Tallon IV. Lock on and try to destroy the shells on their back to make them vulnerable. Once that's done, blast away with Missiles, Charge Beam shots, or Light Beam/Annihilator Beam if you have it.
Fortunately, the Dark Grenchler is a rare creature, only appearing in one room in the game.
