The fastest way of getting level 70 Defense would be Soul Wars 50/50, if it's still going.
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if you have a combat of lvl 70+ It it suggested you kill green dragons they make about a mil an hour
here's the best way to get a high level runescape acount. make an acount train all skills to 70 then train bandits till you have 99 in every combat skill. now you have a high level runescape acount :)
463,885exp
id say kill lesser demons hoopz282
skill levels of 70+ and a combat level of 90+