'Nids are efficient at Melee....but not so much at range. Their shooting phase is usually full of pie plates which aren't very effective or guranteed to hit. Since they're a foot slogging army, stay away from them with anything that has more movement then 6" (12" if the 'Nid decides to run during the shooting phase).

Bikes and Jump troops are the best way to keep out of range while being able to shoot. If you have Krak missiles, meltagun or lasguns keep them on a shooting line behind cover. Missile launchers work best since you can Krak Missile the big guys and pie plate with Frag missiles on the 'Gaunts.

DON'T BE TEMPTED BY HAMMER OF WRATH! If you get into close quarters with the 'Nids they will eat you alive! Your best bet is to stay out of reach and pot shot them all game.

If you want to go toe to toe with the 'Nids (Not advised) then arm yourself with power swords and power fists. If you have anything that will decrease the 'Nids initiative (Psyker or stun grenades or whatever) that will be the best way to make sure your guys can get a few shots in before the Big Nids rip you apart.

The best way to defeat the 'Nids is to stay out of Melee with them. They are probably the worst army for shooting in the universe (Most weapons go a maximum of 18"....with the "Long Range" ones being 24" and they're all pie plates) but they kick ass in CQC. Their toughest psyker abilities are a maximum range of 18".

Of course, it all depends on the type of game you're playing. If you have objectives then you might want to try to out-objective your opponent and win by having more VP's. If you're playing Kill the Alien then stay out of range....