Alone.
You can only get the achievements in every man for themselves (or what ever the option that's not team scoring and not no scoring)
I played it in heroic, with field upgrade, extra grenades and low ammo skulls on, it was a doddle. (first & last levels are probably the hardest to get the achievements on due to lack of enemies)
tbh you can even play on normal and get it fairly easily with a few skulls on. Have a look at some of my scores on Normal with Tough Luck, Catch, Fog, Thunderstrom and x2 silver skulls. Didn't really try anything out of the ordinary just went through on got it. I know Bungie recommend Heroic with several skulls on, but thats making things difficult for yourself.
Please ignore some of the pathetic scores from the 28/12/07 as i was playing very very late at night and deep in conversation - think it was with TorchMT360 and shiftylive?
Definately play it alone too.

