In fact, you might be better off running away to save ammo if you're playing on a higher difficulty. Phantom Necromorphs don't give special rewards for beating them, and instead give standard drops like any other enemy, so there's no need to specifically search for them.

Burn the Undead : Besides dismemberment, burning the infected is another way of quickly dispatching them, with all but the strongest succumbing to a flamethrower within seconds of exposure. Phantom Necromorph Rewards No Special Drops Batman Can Breathe in Space: Due to being undead, Necromorphs are perfectly capable of surviving and thriving in space unprotected with no side effect whatsoever. Phantom Necromorphs are generally the last to appear in a horde, so they can also be considered as pseudo-bosses of a Necromorph attack. They could be considered one grade up from the enhanced variants like the Super-Slasher and Super-Lurker, and generally have twice to three times as much health as the standard versions. Phantom Necromorphs are a higher-powered version of each of the standard Necromorphs appearing only in New Game Plus. What are Phantom Necromorphs? Only Appear in New Game Plus The chart below compares the Phantom Necromorphs' apperance to their standard version.

Dead space necromorphs