Setting unit invincibility

Setting unit invincibility in GalaxyEdit is relatively easy, as Blizzard added a behavior called "Permanently Invulnerable" to StarCraft II. Invincibility can also be set using a unit's flags or a trigger action.

Using behaviors

1. Open up the Data Editor (F7), and go to the Unit section.
2. Select the unit that you are trying to make invulnerable.
3. Go to the Behavior + section on the unit. Double click on the section to open it.
4. Press the Add button to add a new behavior. Now find Permanently Invulnerable behavior, add it to your unit, save it, and you are done.

Using flags

This is another, possibly easier, method for making a unit invincible.

1. Open up the Data Editor (F7), and go to the Unit section.
2. Select the unit that you are trying to make invulnerable.
3. Go to the "Stats - Flags" field of that unit. Double click it to modify the unit's flags.
   
4. Check the "Invulnerable" flag.
   
5. Click OK.

Using triggers

There is an action called UnitSetState. Use this action and set the state Invulnerable to true.