I'm not entirely sure your character would remain unscathed if it stayed on the Mech's back. The A.S.P is only twelve feet tall, so that's a lot of the Pater's body sticking out the back and subsequently being crushed by the stone wall. Not to mention your character went between the Mech's legs, so it had to go around the back of the left leg to get to safety, I doubt it could extricate its entire body to the outside of the Mech's left leg in the half second before the Mech would pincer it against the wall. Especially as it was apparently carrying a rock, which I would presume would slow it down.