Just took down Ropalolyst too thanks to that video. Except I mostly used my Amp (Raplak Prism).
Sargas Ruk is a boss enemy which requires exactly 3 shots to kill him.
If you allow him to attack you his bigger arm (the one he holds up) starts glowing blue. That's where you have to hit him first.
Right after that his chest plate opens and the inside glows blue... that's where the 2nd shot goes into.
And finally you move behind him and shot the blue glowing part on his back.
This fight can be over in less than 10 seconds... or after multiple minutes if you keep spamming him with attacks.
Back when I fought Sargas Ruk, he got stuck in a wall so all of his weakspots were almost entirely inside it. Naturally it took about an hour.