Need help creating a Destiny-ish Nova Bomb

Well, long story short, I am learning how to use Unity by making stuff in it. Ugly stuff, but so far functional enough. My current goal is to make my character create a Destiny-like Nova Bomb (for those who dont know, imagine a huge energy bomb that tracks an enemy and once it reaches it, explodes harming everyone on its radius). The issue is, I created the Nov Bomb and made it track the closest enemy, and explode on contact. I just cant figure out how to make it damage multiple enemies. Any help would be appreciated. Find attached the best picture I could take of my code