um...truth be told...marine armies tend to be quite easy to beat with nids. we outnumber them (typically) and can outdo them in cc. so what if they have 3+ saves, swamp 'em with wounds! also, against marines, they tend to use up alot of points in things like dreads, predators, land raiders or land speeders. with a venom cannon at str 10, or a barbed strangler at str 8, those vehicles should quickly be toasted, leaving only about 2/3 of his points left, easily open for the slaughter. heck, with enough hormigaunts, terminators are easy enough to take down, and even if you don't take 'em out, that's probably 2-3 rounds of them not shooting.
just remember, out number, out innitiative, out attack.