Where in the GWD to kill em and what god item to wear? best tactics to kill em?

wear high prayer armour with a zammy item (like a robe bottom, or unholy book. If you have a fully grown Zamorak Hawk, that is also an option)

Go to the Zamorak area and kill them there. They have the lowest def and hp, so they're the fastest to kill.

Just pray mage and kill them.

 

There's also the option of killing the saradomin mages in the main area but you would need both a zammy and saradomin item, and the task would go a lot slower. The only Plus side is you don't use prayer. But the increased speed of the zamorak mages is more worth it.