This is definitely why Xur exists. Save up strange coins by completing the weekly heroic and pick something up from him... It's the only guaranteed way to get exotics. The next closest thing is loading up on daily bounties, knocking them out every day, and turning them in; it's still RNG but it seems that every time a bounty is turned in it rolls a chance to be offered a choice from 3 exotic weapon bounties. Once you have such a bounty in your possession it is all up to you. Achievable, but it's still quite a chore.