Pros:
-Very tanky.
-Good killer.
-His own flash.
-Running away.
-Stunning.
-Arena ultimate.
-A Demacian.

Cons:
-Very slow in the beginning.
-Bad defense in the beginning.
-Low Mana regeneration

So, if you are battling against ad enemy's, you should take a 'Thornmail' instead of a 'Force of Nature'.
But, if you are battling against ap champs you can take 'Spirit Visage' instead of 'Sunfire Cape'.
Usually you will battle against mixed (ad and ap) champs, you should use this build.
If you are laning against mages, just buy 'Force of Nature' ASAP.

You should take the >Demacian Standard< first because you can throw it at someone for a good distance and then take >Dragon Strike< so you can do that combo, throw a flag and then shoot your spear at that flag to throw enemy's in the air and damage them. After you do that i recommend you to use the >Golden Aegis< to slow them so they can't escape. If they have just a bit health left, just ignite them or flash next to them and kill them.

Your role is to tank. Kinda. At first you are a killer, but later, you are a tanky killer. You can use your ulti to capture your escaping enemy's or to let your Nunu ulti all of the people in your >Cataclysm<. You can also slow your enemy's and tank them for your teammates to kill.