[quote name='hiamiyumi']If you want to get "Immortal" achievement, you have to do it in Multiplayer (4 players do not get killed in one stage)[/QUOTE]
Yeah, I played with my son and got it no problem (just 2 players works). I got him most of the other achievements too just by having him link with me. My experience of playing it myself the night before made the multiplayer *extremely* easy. The only thing he got that I didn't was the "10,000 while linked score" - I guess I'll have to play as him and him as me and I can get it that way

.
It felt kind of cheap to have the achievements be so easy in multiplayer when I got a lot of them in single player before

. The 100,000 and take no damage ones are tougher single player. I couldn't do the no shooting one, though I came close and it, of course, is a breeze multiplayer! Try that single player where you aren't shooting any ships down and have to dodge all the resulting incoming fire (I figured it was like the "Pacifist" achievement in Geometry Wars). Problem is, I think every level has a boss that you have to get through so it won't even work.
As for the 185,000, I'd think you might have to do that in single player (so you aren't sharing points). I got through to the end and got 167,000 or so (was ranked 24th there for a while). It would just be a matter of doing that and taking down more ships along the way, but I think it will be pretty hard.
I haven't tried insane mode yet, but considering that it lets you use continues I can't see that being impossible.