The assassin will auto-counter your first hit every ~5 seconds or so. From there she will do 1 of 4 things: jump attack, low kick, air combo, red dash. Her jump attack can easily be blocked. Jump over her low kick. The air combo is tricky since it’s so very fast, but you can usually block it. The red dash is a little tricky: its two hits. You must block the first hit, and then roll away from the second one. It’s best to chi-counter while she’s dashing. When she is dashing at you she will do a 2-hit attack with a fast animation, so just be ready to block.
She’ll clone at 50% health. The clone shares the same health bar, so try hitting the original and the clone at the same time for max damage. The Buddhist Stomp is great for this boss.