I've literally never played arma ever and got it down on my 5th attempt on regular. The first was just me getting mashed.. the following 3 were learning and changing bindings. Though, with that said, I'd bet most of the kills were actually done by the npcs, as I had a very hard time spotting anyone without binoculars, which don't really help much for aiming.
The npcs did seem to have superhuman accuracy, under the assumption that they too had CQC optics but I never actually looted any of them to check. Spotting I wouldn't say is that unbelievable in the infantry showcase, depending on their equipment. Again, I didn't check to see if they had binoculars or scopes. However the Scuba showcase was pretty brutal. I couldn't seem to find anywhere at all to effectively hide.. it felt a bit like I had a giant neon sign floating around above me.