What are efficient ways to access other scripts ?

Hold references when you create GameObjects of commonly used objects in general. This could be done through a manager class(singleton) or any number of different solutions/patterns. If you have references to your game objects you don't need to ask unity to find them, you can corrolate between the objects by reference or other identifiers on the objects and/or their sub-components attached.

Namespaces are find and don't do jack to you when you're referencing objects in memory, this is a code separation and isolation pattern in it's simplest terms.

Static or class members(as opposed to instance members(non-static)) generally solve an issue that could be better solved with something that isn't global in general. Static methods aren't all that bad, but static variables(fields/etc) on c# classes are generally used to fix something that exists because the person implementing doesn't understand how to manage their code base and takes the easy button way out. We see that all the time here on UA as it applies to holding and displaying scores.

So to reiterate...

When creating objects i know i'm going to be accessing to check properties or whatever about i would probably add it to a manager of sorts, again singleton is fine.

 // Snip
 public int numOfCreeps = 10; // default, inspector changable
 public Creep creepPrefab; // attached in the inspector
 
 SomeCreepManager scm  = SomeCreepManager.GetInstance(); // will create or return a current instance of the SomeCreepManagerClass that is probably a static private variable internal to the class and junk
 
 void Awake()
 {
   // Create creeps
   for(int i = 0; i < numOfCreeps; i++)
   {
     Creep c = (Creep)Instantiate(creepPrefab, transform.position, transform.rotation);
     scm.Creeps.Add(c);
   }
 }

Later i can access the SomeCreepManager.Creeps to collate collisions or whatever floats your boat.