Sendiens' natural bodies are weak to oxygen, so they hide their actual bodies inside of a suit of armor, called an Exoform, in order to kill a sendien, you need to damage its exoform enough for oxygen to reach all of its actual body and dissolve it completely.
The best, currently known ways to do so are:
High perforation and explosive bullets from projectile-based firearms are efficient in making holes in the exoform, although, using this method, you'd need to make a hole in each individual limb until youre sure of the individual's death.
If enough shots are fired, heat from a ''Blaster'' can cause the exoform to melt, being more efficient at creating bigger holes and leaving less room for the sendien inside to hide.
Crushing a sendien's Exoform is sure to at least leave it immobilized, making it easier to kill. Cutting the exoform's limbs is also usefull at disabling the sendien and reducing its combat capability, making it easier to finish off.