Regular enemies fall over like wet tissue, yeah.

Big boss fights in particular (and some of those in particular, in particular, looking at you Maris and/or Gnoster) often have either a gigantic AoE attack that is very difficult to dodge when you're the only source of aggro, or else charge across the entire arena after every attack and have you spend half an hour in the fight, 27 minutes of which was spent just chasing the guy across the map. Not impossible, but very annoying.

But 90% of what makes coop easier in my opinion is just having a spare body around that can pull aggro so you can heal. Elden Ring base bosses were designed around being able to be beaten solo so they usually have big wide windows where you can get a breather if you need it. Nightreign bosses have much less of that because they expect you to be running 3 deep. Nerfing their defenses helps with the solo DPS race but it doesn't really solve the problem that these fights were designed from the ground up for a team who is able to rotate aggro. Nightreign bosses were designed with a sort of raid-boss mentality.

I think if this game were going to be appropriately balanced for single player they would need to go in and edit a lot of the main bosses' movesets. But that was never part of Nightreign's design philosophy and trying to shoehorn it in now isn't doing them any favors in my opinion. This is the equivalent of a World of Warcraft player complaining that they can't solo all the endgame raid bosses. Sure, you can't. They weren't designed to be fought solo. We could try to nerf them down to the point that you can fight them solo, but then it's no longer a raid boss, you've lost the essence of why people wanted to come to this fight in the first place.

Also, are the enemies designed for multiplayer, except in scaling? Everything I've seen looks like standard Fromsoft stuff, no weird abilities that just fuck over solo players.

Compare base game Morgott and Nightreign Morgott and I think you'll see what I mean here. Boss enemies are much more cracked out with longer combos and shorter downtime than in Elden Ring proper, because the developers expect you to be trading aggro with your teammates to give yourself a heal break.