In duels, try to use q,e,auto,w,auto,w, and then ult. Don't use w immediately since the each stack of hemorrhage on the enemy you use w on, the more you can use it. In team fights, attempt to use the same combo on an AD carry or tank. If you get a kill with your ult, use e,q,w on the lowest person alive and then ult. Keep doing this for maximum damage and efficiency in team fights.

Once you have the tiamat, use q and the number the tiamat is on and if they survive, just auto them. Harass with q while farming.

Pros:
•Can take a lot of damage
•Deals a lot of damage
•Can split push easily
•Is great at tower diving
•Is difficult to tower dive and live

Cons:
-Hard cc beats him
-Long range harass beats him(Jayce,Varus,Jinx,etc)
-Uses tons of mana

You should get a primary damage dealer for jungle(such as Rengar or Shyvana), as Darius's e will basically secure the kill if the jungler ganks. This will give a money boost to Darius and the jungler, allowing the jungler to gank better for other lanes, and creating the loop: Ganks,Profit,Items,Ganks,Profit,Items,etc. The lockdown of Darius allows him to gank lanes.