Not Updated For Current Season
For my build, for the skills, I'm not sure of the lvls so I'm not putting it up but max ice blast first and get your ult whenever its available. So Ice Blast>Absolute Zero>Blood Boil>Consume. You might want consume earlier for more survivability. Clarity and Heal are my summoner abilities cause I don't like pots and it allows you to stay in your lane longer. So first get ruby crystal. You can get a saphire crystal and 2 pots instead if you want. On the first trip back, upgrade to catalyst the protector. Then get the sorcerer's shoes, rod of ages and frozen mallet. This can be done in several trips. By this point, you are either winning or losing. get a sunfire cape and warmog's armor. Others are optional.
A good strategy is to ice blast someone and then use your ult. If they are about to run out of range, you can move to cast the ult earlier, but for less dmg.