The episode is titled "Reign." The fight happens at the end of the ninth episode of season 3 of Supergirl. Note: please answer in terms of tactics/techniques.
I don't really think that a tactical analysis really accounts for Supergirl losing here. It's largely down to the fact that Reign takes less punishment and, on those rare occasions that she gets hit, recovers much more quickly.
Kara says "give it your best shot", at which point Reign punches her in the face repeatedly for several minutes. Kara tries to evade and block but is pursued and hit in the face with a shipping container. She grabs Reign and tries to bodyslam her into the pavement but only succeeds in winding them both. Reign recovers faster and proceeds to hit Kara in the face with blunt objects, resulting in what looks like a head injury. Kara evades but gets hit some more. She manages to block Reign hitting her with an object, but only succeeds in knocking her back for a few seconds. Reign throws a car door, knocking Kara into the air and then uses her laser-eyes to blast Kara in the chest, follows her onto a tall building, chokes her and then chucks her onto the ground from height.
Honestly Kara was just never a match for her. Reign is a genetically engineered Kryptonian super-soldier and Supergirl is merely a Kryptonian girl with a modicum of self-defence training.