nieustępliwy ?:

Seles is a tenacious opponent – she never gives in. | 2. There has been tenacious local opposition to the new airport.

My mother always said I was as tenacious as a ferret on crack.

He can be very obstinate at times. obstinate weeds. Invading troops met with obstinate resistance by guerrilla forces.

Korea is unyielding in its demands for a new treaty.

Korea is unyielding in its demands for a new treaty. PERSISTENT, stubborn, tenacious, intractable