If you encounter a mission with an enemy you currently cannot defeat, it's a good time to switch your focus.
1. Patrol the Perimeter - use up your supplies here. They come back extremely quickly and are a valuable source of game cash and experience.
2. Base - ensure your base is upgraded as best you can.
3. Allies - make sure you have the maximum you can and upgrade your Infrastructure
4. Battle - use that Energy for as many combats as you can. Remember, active players often have cash on hand though they are probably stronger than the Inactive players (the "Infected").
5. Gain Levels! Use the above to focus on gaining levels and watch the battle logs. See if there's a weak area (for example, you have a very low chance to hit - this means you should focus on speed gains and computer base upgrades)
6. Refer Others! This is a really powerful means to gain experience in game. Every time your referral gains a level you gain experience which helps with stat gains and HP. Remember also that referrals count as a "super ally" - they also let you add bonuses from surplus tech level 6+ equipment and do not use up an ally slot.
7. Supply Drops - equipment is very important. You can summon additional supply drops at any time for 5 points.
8. SQUAD BONUS: join a squad! Squads provide power to their members.
9. SQUAD WARS: to further increase the power bonus from a squad, compete in the squad wars and place in the top 5! The bonuses can be very big!