You didn’t show/tell how you take damage from your enemies. You could use a bool variable to indicate whether your shield is activated or not.
One thing to note is, it’s not efficient performance-wise to call GetComponent many times, like inside Update - Do this instead:
private GameObject shield;
private bool isShieldActive;
void start()
{
shield = GameObject.FindWithTag(Tags.shield);
}
void Update()
{
isShieldActive = Input.GetKey(KeyCode.LeftControl)); // this is valid because GetKey returns a bool
shield.SetActive(isShieldActive);
}
And then somewhere in your code when your enemy hits/attacks you:
if (!isShieldActive)
{
// deal damage
}
else don’t do anything… 
You can do it without a bool if you want, just:
shield.SetActive(Input.GetKey(KeyCode.LeftControl));
and then:
if (!shield.active)
{
// deal damage
}
I would use a bool.
About the Tags that I used, it’s a class that I always like to use to store all my tags - it’s always better to keep things in one place and not scattered around. Because what if you changed your tag? - you would have to go and change it in numerous places.
A Tags sample would be:
Tags.cs
public class Tags
{
public const string shield = "Shield"; // you can use static instead of const
public const string player = "Player";
}
And then you can access your tags in a static manner (cuz you’re using const), like so:
if (collider.tag == Tags.player)
{
// do something
}
EDIT: So here’s your enemy damage script that you commented:
void OnTriggerEnter (Collider other)
{
if(other.gameObject.tag == "Enemy_Bullet")
{
Debug.Log("Hit");
curHealth -= 3;
if(curHealth < 0)
Application.LoadLevel("gameover");
}
}
Couple of notes:
- As I told you, use the
Tags class
idea to organize your tags better.
- The way you decreased your player’s
health isn’t optimal. Again, we’d
fall in to the same problem: what if
you decided later on to change the
damage amount? you would have to
search, find, replace and update
them all! - So it’s better to store
the amount of damage in a variable.
Where to store that variable? that
depends on your game complexity and design, you
could make it so that each enemy
deals different damage - That way
you would have a parent Enemy
class and then derive from it, so
you have Zombie, Mercenary,
Goon, etc. Or, if you’re dealing
with weapons and such, you could
store the damage in your Bullet
class, so each bullet does a
different amount of damage (9mm,
ShotgunShell, Rocket, etc) - I
HIGHLY recommend you to learn about
OOP (Object oriented programming) -
There’s a lot of resources out
there, for example.
Another thing about taking damage, is that you should encapsulate the ‘taking damage’ code into a method, so you call TakeDamage(enemy.damage) - The benefit of this, is that what if you needed to do other things beside reducing the health? like play a ‘taking hit’ animation, or instantiating blood stains, etc.
Finally, as per your original problem, you don’t need to destroy the bullet, you don’t even need to use bullets and instantiate them - just use raycast shooting. But since you’re using it this way, you could Recycle the bullet, instead of destroying it. More about object recycling (VERY USEFUL)
But if you wanna keep it simple and just destroy it, I don’t see any problem doing that:
if(other.gameObject.tag == "Enemy_Bullet")
{
Debug.Log("Hit");
if (isShieldActive) // remember, use the shield idea I told you about
{
Destroy(other.gameObject);
return;
}
curHealth -= 3;
if(curHealth < 0)
Application.LoadLevel("gameover");
}
What problems did you get doing that? - Did you debug? If not, you should. You can solve most of your problems and figure out what has gone wrong doing so. If you’re on Monodevelop, or VS.