I was level 53 by the end of the game, so didn't have much trouble with any of the Black Ring bosses. I used 1 minor restoration potion for Moriendor, and 2 for the Demon of Lies.
For Divinity 2, the Developer's Cut is the most recent, up to date version.
In detail, there was the original release (Ego Draconis), then an add-on was made (Flames of Vengeance) and a combined release with the remastered original release and the add-on together (Dragon Knight Saga), which was released at the same time as the add-on. Finally, a slightly updated version of DKS was released (a free upgrade to anyone with D2:ED, FoV or DKS), with some extras (soundtrack, design videos, etc) and an added 'Developer Mode' cheat window (Developer's Cut).
Divinity 2 is a single character game, with real time combat, and a third person camera, rather than isometric (the angle and distance are linked in the PC version, so you can zoom out to an almost isometric perspective, zoom in to an over-the-shoulder perspective, or zoom further past the character to look up [occasionally useful when exploring or looting]).
The writing, music and atmosphere is similar. The humour is a step up from Divine Divinity (which was good to start with, IMO). Quest design is also a step up from DD (though it was quite good for the time), but not as elaborate as D:OS.
The world design is a lot less flat, with jumping being a mechanic during combat (optionally) and exploring (sometimes). A couple puzzles have moving platforms, but there aren't any required that are very difficult (some people have had trouble with a couple spots, but adjusting hotkeys or strategy generally helps, or a controller may make jumping easier and there is always the optional cheat window to teleport or morph into a flying creature).
World interaction is less in Divinity 2, though, given the camera controls. You can not collect chests to store things, rearrange furniture, drop items on the ground to retrieve later, etc, though part way through the game you get a storage chest.
Like DD, there are summons, and you can eventually get a creature 'sidekick' (starting about level 10-12 if you know where to go), which you can customize but not directly control.
