The point of the blue build is to get a ton of mana regen and cooldown reduction so that you can spam your abilities nonstop causing a lot of burst damage.

Having Lizard + Iceborn early in game will allow you to farm and clear out lanes with quickness, combined with teleport you can then start to hit other lanes where the enemy team is pushing. Diving into the large pools and launching an all out ability assault.

Ez is always squishy and its probably not worth it to build defensively but a circumstance may arise where you might want to pick up armor against AD heavy teams or magic resist against AP teams or health against mixed teams.

Don't go into the fight first with Ez ever and don't stay in the fight, use your abilities from a distance and kite.