Not much gaming done today, but managed to get done 2 achievements worth 40G.
As I have now unlocked one of the "ghost" characters, the "don't die once" achievements are not that hard at all in free play mode. All enemies ignore the ghost (I presume they cannot see it or figure out they cannot kill it) and focus on their energy on the AI controlled sidekick. So this leaves the main player very safe and only way to fail the achievement is to fall to your death. :)
And if you do manage to get yourself killed by falling off from somewhere, only option is usually to quite the level and restart from the beginning. I had to do it once as one jump to a platform went bit too far and my character fell to his death. :)