We could use another method to check for collisions. Im assuming you want your character to die from the shockwaves when the object slams into the ground.
Imagine you had an invisible background object that overlaps the player only when he is standing on the ground. (or if you have the same ground object you can check if the player "Is overlapping" that ground object).
There is a ton of different ways you could utilise depending on your stage design. If its a flat stage you could check the players Y or when the thing crashes down it could spawn an object either side of it and if that object overlaps the player you could kill him. Just to give you some ideas, the flexibility of C2 is amazing.
For the not being able to move part, if you gave your player an "instance variable" called 'PlayerMove' then you could set that variable to 1 (when required) and then have an event:
PlayerMove = 1 -> Set ignoring input* to true
PlayerMove = 0 -> Set ignoring input to false
*(the input relates to the platform behaviour action).