Just playing around and  i tried to slam as the boss crushed me.
Ended up getting set to the  -integer limit on the next frame.

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FATAL ERROR in
action number 1
of Other Event: User Defined 3
for object Boss:

local variable yy(100004, -2147483648) not set before reading it.
at gml_Script_slam_on_player
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stack frame is
gml_Script_slam_on_player (line -1)
gml_Script_execute_rock_charge
gml_Object_Boss_Other_13
called from - gml_Object_Boss_Step_0 (line -1) - <unknown source line>