When you attack, make sure your attack style is 'block'. This means when you attack something, you gain experience in defence only.
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No, the only chance you get for defence experience is after doing the entire quest you get a 20k experience lamp which you can use for any skill. :)
Proselyte armour.
You kill monsters using the block attack style (trains defence)
No defence level excludes you from damage completely.
In my opinion, defence for higher levels is not that hard, but takes time. Attack and strength are good to raise. I recommend training all of them.