I'm trying to kill Parthurnaax for the Blades... but every time I attack him, his health gets to about 10% or 5% or so, and just stays there... am I supposed to use a specific type of attack to kill him, or is this just a bug?

Don't worry, i'll make sure you get murdered horribly for trying to kill him

Ok, jokes aside i think there were some people reporting a bug on him whenever you tried to kill him even with a quest, he's considered essential, so maybe the game still thinks he his.

had this problem when I got the quest, could not kill him, then i finished the mainquest and tried to kill him again, did not work, but then i traveled back yo esbern and talked to him, after speaking too him again Parthurnaax was killable maybe you should try this?

As far as I can tell, he's unkillable while he's sitting on his 'word wall', and while in the air; and if you bring him to ~0% health while he's in the air or on the word wall, he'll land on the wall if not already on it and simply sit there regenerating health.

The way to kill him, or at least the way I somehow did it, was to attack him, let him fly into the air then land on the ground to attack you, and *then* kill him.

I'm not 100% sure how you get him to attack you, though. When I first tried killing him he just wouldn't die (or even turn aggressive), so I decided, hey, I'll level my archery some on him. After shooting him some 200 times he for no apparent reason turned red on my compass, flew into the air, landed on the ground next to me, then died to an arrow to the face. So I'm not sure what's up with that.

You have to talk to him and tell him you are going to kill him.

I never had that option. All I could tell him was that the blades though he should die, then he started talking some stuff that I didn't pay attention to but I selected all the conversation options and they didn't do anything. I also talked to all the blades and so on before (and after) I just gave up and started shooting him with arrows.

The game can't even bring itself to kill Paarthurnax!

Just use Dragonrend on him while hes on the wall and beat the [censored] out of him on the ground.

Talk to the greybeards....

It might be because you completed the MAIn Quest before killing him. After that he becomes a essential NPC and is Unkillable and you will be unable to do the quest anymo.

He is essential for the main quest so I'd rather break a side quest than the main quest. At any rate, I will go talk to the greybeards to see if I can't unlock a "Kill" dialogue option with him, and then I will use Dragonrend on him.

I did... what you're describing as the solution is obviously not it as I was not able to kill him while attacking him for 3-4 minutes, doing enough damage to kill him dozens of times, then for no reason he suddently died.

I have not killed him, but I think you need to talk to Arnegier and tell him that you intend to kill Parthurnaax. Then you get a lecture of some kind, and Naax will fight back. For now though, level some destruction spells on him

Tell your follower to attack him. It will make him fly up in the air and become killable.

Serves you right, that's what you get for following through on dumb and dumber's request

I went and talked to the greybeards about killing Alduin in Sovngarde, and then about the subject of the Blades tasking me with the killing of Parthurnaax, and told him "I'm still going to kill him" and the greybeards fussed at me a little, and then I exited dialogue and went to go attack Parthurnaax... even using Dragonrend. Unfortunately the few times he took off, he tried to fly backwards, and couldn't (thanks a lot for that bug, Bethesda), so he just landed on his stone again. I wasn't able to get him to take off again once I had whittled his health down to 0. So I will just have to wait for a patch to fix it, or a mod, I guess.

Go back to the blades and make sure you talk to both Esbern and Delphine, make sure that there are no remaining dialogue options. After that yoy should be able to kill Parthurnaax. That's what i did

That might be your problem. If you don't know what's going on, you might have selected an option to not kill him and be stuck with that. Even if you go back and try to redo the conversation.

This is just speculation. I haven't tested it.

When I killed him I was hacking him down with my sword, at about 5% health he used some healing thing and went back to like 50% but I just hacked him down again and he died that time. Maybe it's a bug.