You need three things :
- Dramatic background music. This sets the scene and lets the player know that a battle is about to take place.
Check out Eric Matyas Epic music https://soundimage.org/epic-battle/
- Sound effects. In the video you linked to they used great sound effects. Body hits, sword clashes and swhooshes all add to the atmosphere.
You can find some free ones at freesound https://freesound.org/ . Sonniss also give free sounds every year http://sonniss.com/gameaudiogdc#
You might also consider buying better quality sound effects Humble, Fanatical and Groupe