Just play some rifts and greater rifts, on difficulty, which you can clear fast like in 3 minutes. Look for legendaries which are increasing damage of some skill and get as much as possible of different legendaries working together with one skill and set you are using. That will boost your damage a lot. Check leaderboards to get idea how higend builds looks like and what are the sinergies. Also check builds on this page, that helps a lot. Especialy speed builds.
With barb I recommend you to get wastes set and legendaries increasing rend dmg