OK, THAT’S GOING TO TAKE A WHILE.
First things first: Stretching. “Stretching” is a property of animation, oftenly used at the drawed traditional animation. I usually give “Stretchy” for both of the Arms and, sometimes, the legs. Even though you will keep the node stretched, mind that you have to maintain an equilibrium between the normal node sequence (ex.: arm), basic physics, and flow. Experience will give you this notion. (Actually, one of the “punch” parts looks very weird. Sadly, a good movement might get a very strange pose. You just have to keep faith and follow those frames, trust me 😉 )
Second things second, arcs. “Arcs”, for us, is mostly a roundy pose between the node sequences, like in the example. In the first frame, the red guy has arcs between the body and both of his legs, both with different arcs; and both arms, with the neck. (Pro tip : Draw the arcs in your mind, in a fashion that looks pleasant to you)
Now, some pro tips for general animation:
Don’t tween;
FPS between 11-16 only;
Always stretch;
Mind the flow and the easing;
Take simple BG and floors, just like those Ralph gave to us (love you for that, thou);
FOOTING. Take care with it. Do make the desired corrections for these, even though it gives a lot o’ work;
Stop using FNAF/Undertale/Anime/Scarfie characters. Use a simple yet good base, just like mine (“Fat guy” from Fearios, with some tweaks) and work only with it. Remember: “the best animators uses normal characters, not demons”.

Good luck!