[REQ]How to make an NPC "lock" on?[REQ]

Does anyone know how to make it so after an NPC chooses someone to attack, he cannot attack other people in the area. [ex.In GWD on RS, if your tank fails, and the god attacks someone before tank can attack, you have a new surprise tank :)] I'm making the NPC unaggressive, and you prep for the fight, when ready your tank goes in and attacks first, and is then the only person for the rest of the fight to be attacked. Would really appreciate some help, Rep++++ to anyone who knows. I'm more experienced in client side :(
(Delta source that has underwent brutal surgery)

bumpage