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Targeting is done in different ways by different people.

1) Auto Focus targeting
Set the focus on all your cklones to be your main character ( /focus <main character name> )
Macro all your abilities to use focustarget ( /cast [target=focustarget] Fireball )

2) Auto Assist targeting
Macro all your abilities to assist your main ( /assist <main character name> /cast Fireball )

3) Manual targeting
Make an assist macro and bind it to a key. Hit the assist key everytime you want your clones to assist the main to attack a new target.

4) Focus/Assist sticky targeting
Your clones only changes target when their current target is dead, out of combat, friendly or doesn't exist. This approach allows your main to switch targets while the clones will "stick" on their current target till it is dead. If you are interested in this let me know and I will provide some examples or just search for "sticky targeting" for more information.

5) Party leader targeting
Instead of assisting your main or targeting your main's target you can use the party leader instead ( /cast [target=party1target] Fireball )