I 1ked this game a few weeks ago and may I say that I did them all in pairs, except for Menhir. Axis Mundi was suprising easy once I practiced it a bit. I was really good at the edge mechanic and seeing the best way to make a solid foundation without compromising the tower; while my friend was better at seeing the solutions to the more complex puzzles starting around 115 steps. Once he had a solution worked out, I was able to build the foundation. A few time we had to pull some crazy moves in order to get ourselves unstuck as we both occasionally by accident screwed the other one over by pulling the wrong block.
Menhir I did solo just because it was a little difficult getting two people past all the bomb blocks which in my game were more numerous and poorly placed than any other type. Every time I would get to the more narrow parts and unavoidable bomb block would be in my way on pairs.
Overall I didn't find the babel stages to bad but then again I will say I have a knack for puzzle games been playing them forever. Give a shout out if you remember Fire & Ice on the original NES as that had to be the absolute hardest puzzle game i have ever played.