Start rushing your Essence Reaver in order to get "infinite mana", since for almost every Q you will refill mana even if you hit minions or monsters.

Then, after doing your boots, rush Shieldbow that will get your a healing on basic attacks and an emergency survival that when triggered gets you more move speed and combos with your barrier summoner spell for greater surviveability.

Soon after you will go towards Navori, which when done, allows you to reset your E cooldown on minions and jungle monsters as well, which means you can now Q a minion to reduce E cooldown in order to chase someone, and when fighting a single enemy, your E cooldown will reduce per Q as if you were fighting many.

You can, though, swap your Navori for a Mercurial Scimtar, Mortal Reminder or Lord Dominik's Regard, if the enemy team has too much CC, too much healing or too tanky.

After closing your 4th item, make Bloodthirster. This is actually a late core item of the build that combos with the runes by making your original Overheal barrier even greater and adding to the Legend:Bloodline, Revitalize, Shieldbash combo.

to tie down the build, either you will go for Runaan's, if you have Navori, or you will make Navori now, with 100% crit or go for a second situational item from Mercurial Scimtar, Mortal Reminder or Lord Dominik's Regard.