As soon as the invisibility thing was introduced I found it frustratingly difficult to play single-player. Even for two players, I could see this being a frustrating level, and since there are no other puzzle elements introduced before this it just doesn't set a good trajectory for the game. I would have expected to at least do the "player presses button, other player walks through door" thing once or twice before being chased down by a bot.
Thanks for the feedback! Yeah I was in doubt about this level. I think I'll put it a bit later - it's always good to let the player practice new abilities in a safe environment (I have known and preached this for years - just didn't apply it myself...).
By the way there's no standard "one player presses button, other walks through door" mechanic in this game. The core cooperative mechanic has a similar function, but is a bit more original. It's only introduced in Level 8 however...