I'm not that great, and I am having a heck of a time with this guy.

I've seen videos where the player just goes crazy up close and never runs out of stamina while he keeps this guys stamina in the red and keeps getting finishing blows on the guy,
I don't know how to do that at all.

Even if I'm doing well, eventually, at any point in the match, the enemy guy will do his light speed slide from far away and one hits me to death.
This gets me 90% of the time and even using sloth doesnt help.
Sometimes I shoot kunais at him when he's winding up and it interrupts it, other times he takes the kunai in the face and just does the slide anyway and one hits me, so thats not a reliable way of stopping the slide.

How do you keep attacking him and get all these finishing / grapple blows on him? Any advice is appreciated.

This boss is like a similar set up on a bridge from Nioh 1, and eventually I had to give up and move on.

Scoring a burst counter on any human boss will apply a ki regeneration debuff. Once you do that, it's really time to press the attack.

As for your own stamina, always remember to do the ki pulse. It is a must for all battles. Beyond that, you could try sacred water; which gives you a ki regen buff. I think Onymo magic has a talisman that does the same thing. Having high Courage and Heart stats will also help, but that depends on what weapons you're into and isn't really something you can drastically change mid mission.

As for the super fast attack, you're talking about that one where he spins his spear for a sec before lunging forward? If you can't get the dodge timing down, why not block it? Sure it takes all of your stamina, but it's better than dying. I also think has a range limit, so you could just try running away if you're already a bit away from him. I think you can also stand behind him, and he'll miss. I'm not 100% sure on those last 2, but trying can't hurt.

he doesnt spin his weapon... He crouches and points spear at me, then slides 3/4 of the arena toward me lightning speed, it is unblockable and is an automatic one hit kill on me. Blocking has no effect, it kills me regardless.. Yes I'm using ki pulse but Ive seen videos where the player keeps attacking and keeping the enemy ki in the red almost continuously, doing grapples and final blows. It's amazing to watch.. I'll try barrier talisman and burst counters although those are so hard to time