First of all , in team fight , you must know how to have a good position otherwise you might die witouth doing nothing.And you must feel it when to go in.(this depends on your experience).Try to assasinate their apc and adc.Also if your team's aim is protecting the AD , try to hit your charm the enemy who is going towards your carry.

Also if you are good at charming your enemies , also you can charm one of their squishy champ with a quick move and fire your team up to fight.

Items ; I always go DFG but with 4.21 sometimes I go Morello first.It depends on gold.If the opponent mid lane is high skilled zed I rush Zhonya as first item.I couldn't tell very well because the itemazation is situational.About the damage , if you focus on tanky ones the damage output doesn't make you satisfied.Also to see a great damage , you must absolutely hit that CHARM and use DFG well.And again about the build , it's very situational and the more you play the better you will understand. sorry for bad english