Be aware that if you head into a battle that you're not ready for, and your entire team is defeated in battle, you'll be forced to withdraw from the zone - and you'll lose any special packages you've collected up to that point. Make sure you keep your troops in top condition by training them to restore HP.
Regardless of whether the rest of your troops are alive or not, if your leader falls unconscious on the battlefield, you'll be forced to leave the zone. If you have a defibrillator on hand, you can use it to revive your leader. If your entire squad goes down, you'll need to use a med-evac.
To win a prize in world PVP, you need to win three battles over the space of 10 minutes. The longer you can keep this winning streak going, the better your reward will be. PVP rankings are determined on a daily basis, and the rewards change each day.
If you want to improve your soldiers' special skills, it's important to keep them upgraded. You can do this by "feeding" unwanted secondary cards into your primary soldier in the game's upgrade section.
There are four key stats to be aware of as your soldiers level up. ATK determines how powerful a fighter's attacks are, a higher DEF allows them to withstand more damage, TAC determines the strength of their tactical attacks, while AGI gives each soldier the chance to deal more attacks in a bout of combat.
To promote a unit, you need to combine it with an identical soldier. The end result will be a more powerful unit, at the expense of the one you've sacrificed. Note that you're going to need game currency to complete promotions - prioritize them carefully so you don't waste any of that precious cash.
Keep an eye on your Surplus stash. Very often you'll discover special rewards for service duties, but they'll only stick around for 30 days. Any extra items that you can't carry will also be stored here, so don't forget to take what you need into battle.