Ghost of Tsushima observe is an interesting mechanic and take on mini-boss battles. Observing leaders will give you points to earn new Stances.
Ghost of Tsushima Observe: How to Observe Leaders
To observe a Leader, you must sneak into a Mongol base and get near them. They're easy to spot, as they often wear bright sparkling armor. Hold R2 when you get the prompt to observe the enemy Leader, and you'll earn a point towards unlocking your next Stance.
You'll, of course, have to stealth your way into the base without raising the alarm to be able to observe. Scout the area and plan your infiltration.
If you manage to kill the enemy Leader, you'll get another Stance point. It'll always be worth your time to observe the enemy Leader first, and then take them out with a decisive blow to rack up Stance points.
Stances will help you take on different kinds of enemies easier, as each is suited for a specific enemy type.
You'll need 23 total observations/kills to unlock all of the Stances, and it'll be worth doing that as early as possible by observing the Leaders first. The last Moon Stance will let you make short work of brute and armored enemies like Leaders.