I found that i had to to the challenges alone to unlock attatchments because co op instantly put it to the 3 star mode, now if i have done 2 star on all the challenges can i get the 3 star achievement by doing them all in co op? or must it be done solo?

i only have 3 achievements to go =\ this is one of them

sigh this challenge is stupid ?_? its just luck whether the AI decide not to shoot you, i tried to repair the objective and enemy stood next to me for like 30 seconds not shooting but on another try everytime i touched the objective i was headshotted instantly. friendly AI will not do the objectives it says at the start but they dont even help cover me! will this work in co op?

oh it also says i have 3 AI on my team when i have 2....

Pretty sure you need to do it solo.

Much easier if you do them all as soon as you create your character, the enemy AI is not as harsh on you then

I think that you have to solo the challenges to unlock the star rating (I could be wrong though).
But there's nothing really to unlock at 3 stars except the ability to have your best time on the leaderboards (and the achievements).
If you're doing it just for the weapon unlocks then you can stop after getting 2 stars.

Some people claim it is easier to do the 3 star challenges with new, unleveled characters. I'm not sure if this is true but you can give it a go.

but how do you do 3 star with a new character? if i start a new character i have to do star 1 and 2 again which levels me up =\

Create a new character.
Do the 1 star challenge.
Edit Abilities, Sell. (Rinse and repeat until back to level 0)
Do the 2 star challenge.
See above...

ahah! ok thanks! does it make a massive difference?

Definitely. You'll still probably [censored] about your team's lack of intelligence sometimes on 'Be more objective 3stars', but everything is still a LOT easier...

Some tips :
- Be more Objective / 1 and 2 stars are very easy on level 0; 3 stars just straight run to the first objective, pull back behind the wall if you haven't been knocked down yet, and if the enemy disables your HE charge, restart, you most likely won't be able to put it back up (I *NEVER* was able to, seriously!). The last objective is pretty hard (hackbox :\); try disguising as an enemy, wait for most to be downed by your team, drop the box and then go hide by the command post. Usually works. The second objective (repair) can also be made alot easier by working your way counter-clockwise on the top level, instead of rushing straight for the objective... coming upon the enemy from behind can make short work of them, especially since your teammates will definitely be coming head on, creating a crossfire......
- Tower defense / 3 stars is similarly hard once you reach the 3 waves step (3x3 = 9vs1). I usually take a shotgun or lobster as my main and rush one of the flashing lights corridors as soon as possible. If you can kill one (sometimes 2!) of the teams before they can spread out, it all ends up a LOT easier... If you die, don't forget to buff your gun!
- Escort / If your bot ever gets destroyed, use it as cover to clean up. Kill all but 1 of the bots (important!). Then go back to your bot vehicle, repair almost all the way trough, but not quite, kill the last bot, repair the bot vehicle, and start moving again.. you should have a few secs before the next wave comes by again... If you die, don't forget to buff your gun!

Edit - Update to make things abit clearer?

thanks for the tips! i shall give it a try!

when hacking you can stand round the corner near the com post. you dont have to be right next to it. that really helps

also, use smart to climb the structures rather than using the stairs near their spawn. much easier!

i did 3* at level 16 and at level 0 and 0 was a bit easier. i didnt find any of them hard though.

Hmm, now that I think about it, I think the BMO tips I gave here were for 2stars... I had a hard time getting trough that one because of the hackbox, and in the end found 3 stars to be easier than 2... (IIRC)

I have 3 on 4.
I miss Parkour (2 stars), Im not so 'agile', so
The hardest one for me was Tower Defense, the other 2 I found them easy...

My one gripe with TD? There's no overtime if you're already capturing the command post when your timer runs out... Got my ass kicked by the last wave (9 guys); came back in, killed everyone left, then capped the post... only for my timer to run out about 1-2 secs before I was done. Darn.

I found escort the hardest. I may have been lucky on "Be more objective" 3 star when I did it.

be more objective is easy when you know the routes. i did all 3 back to back. there are some easier ways of doing them when you know alternative routes up the scaffolding.