Try to disable "left" and "right" controls while player in air.
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In 8-th event, when monster is overlapping waypoint I changed the 'Set bullet angle of motion' to simple 'Set angle of MONSTER'. Seems to be working, try it, maybe it's what you need.
I suggest to use the initial layer (layer 0) for back-back-baaackground only and create new layers for your game right in the beginning. Name your layers and make sure that they're in right order (on "layers" panel in top right of C2).
Tried to solve: dl.dropbox.com/u/66358591/physics_collision.capx
ps. click couple of times on 'mode change" text, I messed something up with it a little bit
Sorry for off topic, but that's my favorite thread on whole forum.