I would say what makes it hard is the unskippable cutscenes and the glitches you can run into on a hardcore playthrough. I got my achievement on my 9th playthrough. I would run into 1 glitch every playthough such as skipping the cutscene where the swat team is breaking into the train (Freeze) or switching to a new weapon just before a cutscene that would make 2 of my guns clip into each other (Can't Shoot). I would say your best bet is to know the spawn points, painkillers and practice on certain parts you die on. I never used shoot-dodge (Bullet Time). In most cases you will need to RUSH your enemies first. If you keep at it I'm sure it will get easier but if your on your 20th attempt and still getting rocked then maybe it's time to let go.
If only they had a save point at the disc swap or maybe something like Dead Space 2's save system with 3 saves only.
I just completed NYM Hardcore last night and it was one hell of a ride. I just want to make a club where people can post their NYM Hardcore times with a picture taken after. Goodluck for all others going for this dreaded achievement.