Easiest done on Normal.
Although, there are parts that are rather annoying. The civil war that Artyom runs through during the main campaign is actually more infuriating in places than Artyom having to fight Nosalis'.
The best advice is to use quick saves on full health, discover where enemies are, if you take damage, reload and kill them then save again at full health.
It's also worth noting that I've read on some websites that chaging the difficulty on a Ranger Hardcore savefile to Normal doesn't actually change it to normal, so it's best to start again on Normal and then quit out of the main campaign after the initial introduction sequence.