How to Detect if a group of NPC's are dead?

i am making a Wave based game with this i mean that the player killes the NPC's (about 10 clones) then a "wall" becomes collide able so that the player can walk over to the next station.

here is my script:

-- make the detect Killed NPC thing here! i think?

local ten = script.Parent.ten
local nine = script.Parent.nine
local eight = script.Parent.eight
local seven = script.Parent.seven
local six = script.Parent.six
local five = script.Parent.five
local four = script.Parent.four
local three = script.Parent.three
local two = script.Parent.two
local one = script.Parent.one
local Kill = script.Parent.Kill
local part = script.Parent

-- make the if killed then or at beginning 

part.Transparency = 0.3
part.CanCollide = false
Kill.Transparency = 1
ten.Transparency = 0
wait(1)
ten.Transparency = 1
nine.Transparency = 0
wait(1)
nine.Transparency = 1
eight.Transparency = 0
wait(1)
eight.Transparency = 1
seven.Transparency = 0
wait(1)
seven.Transparency = 1
six.Transparency = 0
wait(1)
six.Transparency = 1
five.Transparency = 0
wait(1)
five.Transparency = 1
four.Transparency = 0
wait(1)
four.Transparency = 1
three.Transparency = 0
wait(1)
three.Transparency = 1
two.Transparency = 0
wait(1)
two.Transparency = 1
one.Transparency = 0
wait(1)
one.Transparency = 1
Kill.Transparency = 0
part.CanCollide = true
part.Transparency = 0

this script shows that if the player has been killed then a Timer starts and the door/wall will open

i need this script to be modified! i don't understand what to do if you tell me what to do else if you tell me very detailed!