Things to remember:
When helicopter flies in and starts shooting, its a good idea to help it. The longer you take to kill a hive, the stronger the monsters will become. You can reach end-game monsters at Hive 8, like my team did just now. Like 4-5 invincible monsters from all directions pop up and they don't take any damage at all... unkillable. By the time you kill one, there are at least 3 more on the field.
Failing challenges is a good way to screw up your team and make sure you never win. Especially those at the beginning. I played with several teams that do the following:
Challenge says "use only melee" and the first thing they do is shoot a bullet, THEN they use melee. If I see such a player, I leave immediately (if prior warning about challenges was made). I tried to tell them, I tried to do more than 1 challenge with them, they just don't care. They think their gun's biggest, their muscle - strongest and, well, we'd lose before we make much progress anyways. Its a team game, don't screw your teammates.
If you just like running around shooting stuff, go play multiplayer. Seriously. Extinction is a 4-player CO-OP game, teamwork matters more than the amount of bugs you kill. Statistic isn't saved. Wanna run around and shoot something - pick a multiplayer/single player game, don't screw public CO-OP games.
When a "have XX% accuracy" challenge appears, its a good idea to stop shooting like crazy and use single-bullet weapons (like pistols) and aiming well before you shoot.
Other simple challenges include:
Kill only in melee
Take no damage for a period of time
Do not reload for 60 seconds (just use knife, or run away)
Kill 25 while prone
Kill 5 in melee
Find and kill this monster in 30 seconds
Kill this monster in melee (shoot a few bullets and finish off with a knife)
Melee means knife means pressing E button when enemy is near. Most likely, more than once.
Going prone means lying on the ground (default button: CTRL, or look into your settings as it may be different).
Keep an eye what everyone upgrades (armor, bullets, explosives, bonuses ect). So that you don't spend skill points on the same thing. Not sure? Ask! Players don't bite, unlike bugs from Alient Swarm (oh did I just open the big secret here?)
Try to win challenges, as its the only most important thing you absolutely need. You'll never have enough to upgrade all skills.
Never fail challenges. Funny how simple this is and how many people actually ignore this point. They just don't care! If you don't care, don't play Co-Op mode! Play free-for-all multiplayer!
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You won't believe how glory-seeking some people are. I enjoy playing as engineer, so I had that up when I went into a lobby. When it loaded, all three others were also engineers. Like me, they probably figured they would play it cushy, setting the traps, protecting the drill, always having bit of extra cash. After a few seconds none of us changed to I switched my class to medic. I was expecting at least one of them to switch to a wep spec of another medic or something, but they all started readying up. None of these three people were willing to swap classes. I don't understand people who do this. The entire point of online extinction is that you need to use teamwork to get your entire team out. What are these people expecting to happen? That the other three will carry them to the nuke and back?
Let my ******** be a lesson to you: no matter how good you are in single player, the only thing that guarantees success in co-op is teamwork.
Also, have at least one person in your team max out their death machine before the second barrier hive (before the first if they can spare the points)
i agree with Thanat0s_ZA
and please let the engineer take the drill......( in order to have 125% ) many times i get in lobbies where i was the only engineer and the other players always try to take the drill from one hive to another....