In MW3 Zombies, you always need Essence in large amounts to buy and earn the best rewards. Here's a fast, efficient way of farming it without scouring the Exclusion Zone for missions.
How to get Essence fast in MW3 Zombies
Normally, you would obtain Essence(in large amounts) by completing Missions and Contracts while running around the open world. While this method is effective, you risk dying often and losing your gear doing so. You also are forced to constantly roam around the map, either on foot or with wheels. These things can be tedious to do if you're aiming to farm up Essence specifically.
A much easier (admittedly less exciting) method is to farm Exfil calls. For those who are unaware, when you call it an Exfiltration chopper, a very large amount of weak zombies will continuously spawn around the whole area. These zombies are slow and can be easily killed with just about any decent automatic weapon. Killing zombies also earns you Essence, so while each individual one rewards in small doses, as a whole, you'll get a lot of the stuff very quickly. Before you do this, though, I do recommend looting some decent gear or bringing some in, all to make the process a tad easier.
When an Exfil is coming to an end, do not get on the chopper. Instead, stick around for the mayhem to conclude and wait a minute or so to call in another Exfil. Exfils have a really short cooldown(shown as a grayed-out version of the Exfil icon on all maps), allowing you to abuse the guaranteed zombie spawn from them with ease. Since Zombies also drop ammo commonly, you never need to worry about that either if you're placing your shots well.
For more useful guides, tips, tricks, and information on all things Modern Warfare 3, check out How to find Mystery Boxes in MWZ (MW3 Zombies) right here on Pro Game Guides.