C# / JS work with game being developed in unity 3d

I have a game I am making (prototype right now) and one of the tasks that needs to be done is I have a 3rd person character that I got from the assets store and I am using that as the player then I added another one now I need to take the other one and declare it as a enemy and have it attack if the player gets within so close to the enemy character but stop attacking if the player moves away. There is other tasks but this is a important one at the moment. ASK ALL questions before bidding anyone over the allowed range will be ignored. If award I will send project to you