Just try what my guide says..

Ohh and sell your wriggle's Lantern and buy The Bloodthirster instead.

Pros: Extremely OP
Very good ganking ability
Has alot of hp late game
Does tons of dmg with his ulti + good for ksing..
If you succsesfuly gank 3 times.. Its done for the opponents team.

Cons: You need to know Jungling before you try it with darius
He has limited ability to escape fight. (althogh he almost never does)

When you approach a gank you need to try to hit Q in the right range for the +50% Bonus the skill has if you attack from a fixed distance. try and hit crippling strike right after you basic attack atleast once because crippling strike resets your basic attack time.
when the crippling strike effect ends then you want to hit E and pull them to you and then land another Q Basic attack W combo.

Ask for a team mate to pull the golem then hit Q (try in the bigger circle for extra dmg) and kill him with smite. Go to wolfs next do same thing at wolfs with your Q basic attack W combo,
then move to wrathes do same and then go to golems you know what to do and then go to red
kill him with smite and search for ganks :) .