Pick him against low to mid range champions with low mobility and whose method of CSing stops their movement, like 
Pay attention to your own minion waves HP.
Whenever they stop moving to throw an auto or spell at a creep, throw Q for a bomb with a high chance of hitting.
Also get some flat AD marks on Zilean. His ability to last hit under tower (from my experience) is really poor when under his turret. The usual "2 turret shots and 1 auto attack" trick doesn't work for him.
Or just play a better champ.
I like zilean as well, only issue is at higher elo you will find it very difficult to land bombs without using slow initially. His combo is way too mana hungry pretty much forcing you to use 2 item slots for mana. If there was an ability put in such as refund certain amount of mana on stun or make his W no cost then I would go back to playing him more.
I know there is a rework of his passive in pbe at the moment, but that doesn't really help a solo lane zilean
