I've never had trouble with Nito going solo. Heck, he's the easiest of the 4 bosses in my opinion. I also never run into invaders there, it's mostly in the catacombs. My StrategyJust stay back at the start with a divine weapon and your main weapon in your off-hand (as in ready to swap with said divine weapon). The big guys in the back don't aggro unless you run up to them, so with a divine weapon you can perma-kill those first small skeletons, and Nito by himself is just a joke.

Dukes Arhives is a pretty area I will say, the sense of scale there is amazing... I just hate the channelers, the archers buffed by channelers, and just everything.
Since I barely ever up my vitality until end-game/new-game+ it was still around 20-25 when I went there, so the archers were near one-shotting me when buffed. That wasn't an enjoyable experience.

Tomb of the Giants is still really tense, but I know what enemies I can avoid, and what enemies I can run from now, so I can clear it in like 5-10 minutes from the start.
The one dog skeleton you have to fight makes me so nervous though, I one-shot him, but he can also one-shot me with his endless combo of doom and destruction.

Lost Izalith isn't hard, it's just long and annoying and red... very very red.
The bosses there look sweet though. Seriously some of my favorite bosses in the game aesthetic-wise.