Crota actually got tossed into the wound where the Vex were (as punishment from Oryx for f---ing up*), and made his name fighting through eons and eons of Vex simulations. The Hive are hundreds of thousands of years old (I think), so Crota potentially could have killed millions/billions of Vex enemies. *Oryx says something along the lines of, "Fight your way back through time or die in the process."