It's pretty easy once you've levelled up, I completed it on the weekend as well. I only died once during the normal mode anyway.
You can run through NM mode very quickly if you just concentrate on the main objectives.
Make sure you level up plenty by doing all of the side missions on normal story mode.
Yeah that's what i did, it only took a few hours. Level 50 with maxed out stats it was very easy.
I did have a few people join now and then who wanted to mess around, but i just booted them if i got to a point where i was 'waiting for other player', no time to do that in nightmare mode really.
I think just doing the main missions and the odd side mission i had maybe 6 hours left on the clock at the end.
. I only died once during the normal mode anyway. 
), but the replacement controller managed to survive the rest of the battles. As a result of being so impressed by the game, I have just ordered Tales of Graces F, and am considering Vesperia too. I'd like to get the Japanese version, but I think it would seriously hinder the ability to understand the game content. Not so much the story, but the item specific text. Not knowing what a weapon does, or what spells do, could be a bit of a pain.