Kalusk possesses strong shielding and armor, and like any krogan, has krogan charge and regeneration.
Kalusk is the most powerful enemy encountered in the assignment. Upon entering the final area of the facility containing the generator, Kalusk will attack. Keep away from him and watch for a krogan charge. Eliminate other enemies first and then focus on Kalusk; by himself, the damage he can inflict shouldn't be serious. After Kalusk's shields and armor are broken, it should be easy to finish him off.