If you find yourself as the person frequently dying when the Zerg on Zerg warfare erupts here are some helpful tips.
Tip 1: Formation
Here is a how many battles go ...
VVVVVVVVVVVVVVVVVVVVV
VVVVVVVVVVVVVVVVVVVVV
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The people who die all the time are the ones that keep YOLOing into the
line and STAY THERE. The people who survive big battles are the ones who dive in and out. If you don't use your friends as meat shields (and you're not a tank) you will die.
2) Look around
A large fight is not 50 individual 1v1s. If you are focused on just one guy you will die.
3) Stay together
Remember how Jon Snow looked when he was surrounded by zombies on his ice platform? That formation will keep you alive because, don't forget, there are two enemies, and not one.
4) If it looks like a trap, then it's probably a trap. If it doesn't look like a trap, then it's definitely a trap. So don't go running into traps. Every person needed to res you is one less person to fight the bad guys. Similarly, res EVERYONE. One more guy to fight is one more guy to take the damage that could otherwise be pointed at you.
5) Take note of your surroundings. Just because it's a big open field doesn't mean you can't find objects to help you find shelter.
6) Eat your food and repair your gear before you fight. Nothing will screw up a potential victory faster than running out of food the moment you start swinging your sword.
These tips are very basic and someone reading them will undoubtedly say, "well duh", but they will prolong your life in Cyrodiil. None of them cost gold or require new gear. They are just basic blocking and tackling tactics that some newer PVPers might not yet get. Hope this helps!