Scratch

You could try to make the ground sensor time how long it takes for the player to land (as in while it's not touching the ground) and if it's over x seconds, the damage is applied.

You can either build a counter into your fall loop (see below) or use y-velocity and say "if (y velocity) > (some number), then die.

When I receive [begin to fall v]
set [fall v] to (0)
repeat until<touching [ground v]?>
change [fall v] by (5)
change y by (-5)
end
if<(fall) > (20)>
blocks you use to kill sprite go here
end