Bull zombies are distinguised by their black riot police gear. The easiest way to deal with one is to tase him with electricity so you render them useless for a couple of seconds, allowing you to exploit his weak spot which is his exposed spine. If for whatever reason you are unable circle around a Bull zombie aim for the riot helmet. If you break the glass mask, you will be able to down them with a headshot.
Don't Straff
The best formation when dealing with a horde of zombies is to maintain a line with your teammates. Zombies will not fire back so you don't need to strafe around and risk taking friendly fire.
Hazmat Zombie
Always look for a headshot from afar when dealing with a Hazmat zombie as a clean headshot will prevent them from exploding and releasing green toxic gas. Hazmat zombies are distinguised by their yellow hazmat suit.
Leading the Team
A good tactic if you find yourself leading the team is crouch when zombies appear. Doing so will ensure you don't get in the way of your teams line of fire and getting shot by friendly fire.
Practice
Although World War Z is an online game where you are part of a team of four players you can if you want to begin with play with bots until you feel confident enough to go online and play alongside real people. Even on the lower levels you will find that the bots make effective teammates.
Reconfigure Automated Turrets
Take advantage of Automated Turrets by reconfiguring them to attack the zombies. The player who sets the turret will recieve the kills. To hack an Automated Turret you just have to get behind it.
Screamer Zombies
The Screamer zombie will start screaming as soon as they spot you which will alert other zombies in the area. Make sure especially during the early game when you only have basic starter weapons that you kill the Screamer zombie as quick as possible. Screamer zombies are distinguished by their yellow construction helmet, orange vest and blue pants.
Stealth
An effective tactic to complete missions is to employ stealth by having the team only use silenced weapons. By moving through areas without arousing the attention of a horde of zombies will make completing your objective easier.
Zombie Pyramid
Whenever you see a zombie pyramid created by the Swarm zombies target the base. If you aim for the base and destroy it the whole tower will collapse.
Bull Zombie...