There are some "superman" scripts in the Ed Depot that might help.
You can edit the game config or something to make units stronger.
With pure scripting, it's difficult. You can easily make an invincibility script and mine is posted below. You can amend it as you like, you'll probably want to change
_x setDammage 0
to
_x setDammage ((getDammage _x) - 0.5)
or something. Syntax very ropey. However, in certain circumstances you will still be killed. If you do use/amend it please mention where you got it from somewhere in the credits.
; this script is for beta testing and review purposes only
; you can call it from the top of init.sqs
; it makes the player's group unkillable
; units sometimes get killed and are resurrected
; if you get killed that's it, you are dead
; even with this script running you still have to play tactically... if you just run around like you really are invincible you will die
; sn1 is the player
"_x addEventHandler [{Hit}, {_this select 0 setDammage 0}]" forEach units sn
; it looks fine on paper but in practice the killed EH doesn't help
; when your loons die they get resurrected but not in your group
;"_x addEventHandler [{Killed}, {_this select 0 setDammage 0}]" forEach units sn
~2
hint "Invincibility script on"
#loop
; if you don't put your loons in the setdammage loop they can get killed
; sn1 setDammage 0
;"_x setDammage 0" forEach units sn
"_x setDammage 0" forEach [sn1, sn2, sn3, sn4, sn5]
; uncomment the setCaptive line if you want the enemy not to shoot at you
; it's in the loop because the squad is setCaptive true/false during cutscenes
sn1 setCaptive true
;"_x setCaptive true" forEach units sn
~1
goto "loop"
exit