Zeratul is now one of the most broken assassins thanks to balancing efforts by Blizzard. Buffing his auto attack seems to be the best way to go. At lvl 13, pick between mending strikes and spell sheild depending on

A)If you have a healer or not
B)If they have high magic burst dmg

Shadow Hunter has ELIMINATED his mana problems giving your Blink maximum uptime AND costing you a mnere 10 mana for your most important ability!

Be advised that Void Prison is still possibly the strongest ultimate in the game, experimenting with Shadow Assault is not advised at this time