go to the foyer bonfire. backtrack towards the second bonfire untill you get to the spot where you find the first enemy who has the back turned. kill him and deal with any other enemies coming from the ash-area. At this point don't climb up the ladder but turn around again. You will see a staircase on the left of the path which you took to get here. Go down there and walk accross the chain. At that point you will be invaded by a nasty Dark spirit with a great lance, who hides behind a corner on the top level untill you get there.

Waaaah

Awww poor precious baby.

Force the invasion, restart the game. He won't respawn until you sit at a bonfire again, so, no more ***** NPC invader roaming the tower.

Rush Nadalia, take her out with a smelter wedge, then sprint back upstairs and slowly descend. You'll eventually get it done!

Once you've done it, you can sit at a bonfire and get back there to fight the invader, keep in mind you'll have to kill everyone else as well, again.
Good thing is that they no longer have ash, so they're weaker, and none will "revive".

You can also just walk back to the foyer bonfire and then just head a bit further, (like you're going to the room with all the barred walls you can raise,) and he'll despawn on his own.

Just because an area is difficult doesn’t mean it’s “*******”

It's not difficult, but it is indeed *******. It's also monotonous, boring and dull.

basically it's best to keep one so you can fight with "enough space" in one of the corners so that he doesn't heal

Would've been nice if the guide said that before it told you to use them all before.
What an atrocious DLC

There are ten smelter wedges in the level, and ten ashen idols, including the four surrounding the Fume Knight's arena. You have to go looking for the extra four wedges, as they're secreted away, but their location is clearly listed in the optional content section of this page,