Geez... you'd be difficult wouldn't you?
You want it to kill parachutist's too do you?
Nah, only joking mate ;D
You're right, the trigger before isn't suitable for that... it will only activate if the number of units in the trigger radius changes (which it doesn't if someone parachutes in).
So, try it with the following modifications:
TriggerRadius:
as requiredCondition: Anybody Present (repeatedly)
Countdown - Min: 5 Mid: 5 Max: 5
Condition field: count thislist != noflytrig
OnActivation field: "if (getpos _x select 2 < 2) then {_x setdammage 1}" foreach thislist; noflytrig = count thislist
OnDeactivation field:
noflytrig = 0
That will also activate based on time (5 second intervals) if there are any units in the trigger radius.
Hence, it will be going off every five seconds, so when your parachutist hits the ground, they will have five seconds (if they're lucky) before the trigger gets them