I just got that game and as much fun as it is (most fun xbl arcade game I've ever played, better than most games I spend 60+ dollars on) the online coop is tough as hell whereas single player is, as you stated, easy because you've got infinite lives. The more people you have playing, the more enemies you have to face and the more hits the bosses can take. In addition to that, you only get 20 lives each and although that sounds like a lot at first, trying playing through on a much harder difficulty. The furthest Ive gotten is to the bottom of the elevator on the giant shredder dome thing. Multiplayer is a lot more fun, the whole point of the arcade game was to play with other people, but I think they should just give me the ability to enter quarters directly into my 360.

The multiplayer achievements for that game are also pretty tough, but I agree with what you said about the single player achievements. By the way the secret single player achievement is [spoiler]falling down the manhole 5 times[/spoiler] but it gives you no gamer points :lol:

PS That robot boss that you fight to free splinter is a douchebag