If you have not completed Dom Upon All The world, then you have not completed the game on nightmare so, yes lowering the difficulty would nullify the achievement. Assuming you play on PC, you could cheat to get through that fight. Check out Cheat Engine and the DAI table 45 for all versions. Actually it isn't for all versions and, most of it doesn't work or causes problems now but God Mode still works. Use it to get through that one fight, then, exit the CE console and go find another fight to burn off the guard CE gave you as well as get rid of the barrier it provided before saving. Saving with that guard or barrier can cause it to become stuck for the remainder of the game so that no one that was in that party ever looses guard or barrier.
It's never done that to me, but I've read of it happening to others. Just search, read the directions on the CE site and, don't use that cheat table except for that one fight to get the achievement if you must. Also remember that CE is not a mod, but, it is still a third party tool and, no one but you is responsible if it crashes your game or corrupts a save. (never happened to me and I use it for the whole game if I want a story focused game on that toon.) Just don't use most of the table, some of it works but, some of that now out dated table (but still the most recent with God Mode) crashes the game or simply does nothing now.
@rschenkel89 @DarkAmaranth1966 Be aware that using Cheat Engine (especially the God Mode cheat) has been known to permanently break characters, I had this happen to me accidentally when using it for the Party Banter trigger, I forgot to remove/disable the 'God mode' hotkey and accidentally pressed it (or... my cat did, because he really likes flopping all over the keyboard 😛 ). Use it at your own risk, and save before doing so. If you're on PC, there are several extremely cheaty mods which can make you nigh unkillable and are much safer to use (though may require a bit more effort).
Alternatively, you can leave the DLC until after you've finished the main storyline, then it won't matter if you lower the difficulty, as you'll have already earned the achievement if you're eligible for it.
