Zagara is one of the best specialists in HotS. She can put a lot of damage to structures and minions and with this build she can do even more! However, she is quite slow without Creep and assassins will kill her immediately before she gets tougher on higher levels.

1 Always remember about Creep. A whole lane should be full of it, otherwise you won't escape when in danger.
2 Envenom is you friend! Most assassins have low HP, so with Envenom they are likely to back off. Assassins who can't put a lot of damage in one second ( Lunara , Tychus , Valla ) will give up in most cases. However, you should be afraid of Zeratul and Illidan
3 Nydus Network - we can just be everywhere! On big maps like Sky Temple it's very useful. Just put one in the base, two near temples and one near a merc camp. Of course, when you solo lane you should always have a Nydus nearby. It will save you and allow aggressive positioning.
4 Hunter Killer - Most ememies will be afraid of Baneling Barrage and try to dodge them. Although they look scary, they won't give a lot of real damage. They're also easy to avoid. Your true source of damage is Hunter Killer. With it, you should duel without problems any hero, especially on the highest levels ( Zagara gains a lot of power in the later phase of the game).
5 Infested Drop - pretty good against minions. It's also good to use while escaping (you don't need to stop to send some Roachlings

SOON I WILL CORECT THE DESCRIPTION OF THIS BUILD - I'M GOING TO WRITE SHOULD YOU TAKE THESE TALENTS :D