Played extinction a little, learned the mode a little, hopefully this info will be useful to you:
1. Challenges are a MUST. Each completed challenge gives you 1 extra skill point. They help a lot! Do whatever you need but get that challenge completed!
2. Challenge is made for EVERYONE! So if challenge says "use only melee to take down aliens", use only melee! This applies to everyone on the team! If someone fires a bullet, you're ALLbusted. If it says "keep accuracy at 75% or better", only fire when the enemy is at point-blank and not moving. Only with pistols or shotguns! Every missed bullet makes it MUCH harder for the team to keep up with the challenge. Use melee!
3. Using money. Press 3, 4, 5, 6 to buy stuff. Yes, it shows "press up down left right", but it actually means "press 3-4-5-6". Funny how it wasn't made to properly show bound buttons and instead shows gamepad controls.
4. Upgrading skills. If you complete a challenge, you gain 1 skill! You also gain skills for destroying hives. You can use skill points to upgrade various abilities. When you drop an ammo box, you get ammo for yourself as well. Same with explosives and other bonuses. First skill (class) only affects your own player, not the entire team, so upgrade it last.
5. Earning money. You earn money for killing enemies, assist kills and by finding it in boxes or garbage.
6. Spending money. You spend money on 3-4-5-6 buttons (up-down-left-right, ammo boxes, IMS, turret, support boxes) and stuff like traps and guns.
Feel free to add any tips or tricks or make your own input.