whenever i go for achievements, i definatly tell the people what im trying to get and if its ok, aegis wing for example, i hooked up to a friend and got about 3-4 achievements in the first level.
people like this kid your talking about are retarted to put it bluntly.
and all the achievements in this game are really easy and can be done in coop mode except ACE.
if your having troubles send me a FR. ill hook you up.
my GT = Ho5tiLe D