It's pretty extreme for an idle animation. Looks like he's saying "Nyahoo" repeatedly- I agree with the review below observing its like he's being surprised over and over again. Heavy breathing for an idle animation is great, but this guy looks like he's announciating something with a lip-sync- it doesn't read as a breathing loop.
Additionally, with the size of the gun and the position he's holding it in, even if we were looking at him flatly from the side, the barrel is dipping too far below his feet. This would work in an isometric view where you have a plane above and below the character, but we can see contradictory information- the camera is aligned with his head, we can see that his ears are at even height with each other. That's not isometric alignment, that's just a character on a flat, side-scrolling plane. My point is: moving forward you should clarify and commit to the decision whether or not this character is being viewed slightly isometrically, if the gun could be smaller, or could he be holding the gun in a different way so it's not clipping through the ground he's standing on?
The animation is great, especially on the head and face. Fabric wrinkles look fine, his knife-arm elbow is accentuating how wildly exaggerated his motion is, and the forward heel lifting and lowering over and over again seems more distracting than beneficial to the movement you're trying to sell here.
I look forward to seeing what you have next! Keep up the great work, reach out and take it.