There is a thread on rs forums that describes a theory on how you get a defender quicker. He said that if you only camped out and kiled one everytime it respawns you won't get anywhere. But, if you can find an empty world and kill all the giants without anyone interupting you you will get a defender insanely quick. He got the idea because the girl you see infront of the door of the room with the giants says "One of these giants has the next defender you need." Meaning, one of them has the defender at all times. The reason at sometimes they work when camping, sometimes someone kills the one that your giant has and then it gets transfers to the one you are killing. To some this may a confusing theory but remember, it's only a theory. It seems to work for me so I beleive It.