This build is very strong at level 18. The ULT is 90 seconds, drops to 14.4 seconds if you have any mastery and runes correct points, and the skill Ice Blast, 6 seconds cooldown, will drop to 1 second, ie if their enemies feed you, will not be nunu tanker, but a killer.

early in the game you can go anywhere, I recommend mid to creep
after level 10 you need to go jungle to get mana regen, because you will spend much