Here's an old 'guide' I wrote a while back for Challenge #6 on another site, 6 is considered the most difficult to most people, I did 6 my first try and I found 4 to be way harder personally, I actually took about an hour to finish Challenge #4! But since most people struggle with Challenge #6 however, here are some tips from me to get through it easily
Wave1: Very easy, if you care about your time try to rush the spawns but regardless if you rush or not the strategy is simple, pull out the shotgun and just roll towards and enemy, enter AR and fire, keep doing this, you can easily take out all the enemies really quickly, as for the boss guy, keep your shotgun (you'll probably have like 5-8 shots left but it doesn't matter) and just roll around and shotgun him, pick up more ammo for the shotgun and keep going, then when he is low on health find the LFE gun that was dropped (just makes it faster for later on for a good time) take a shot with the shotgun and once the wave is over pick up the LFE and go to the far left side for Wave2
Wave2: Wait for the enemies right in front of you to almost spawn, throw 1 frag and shoot the LFE and run to the main stairs from the start and go behind the cover, wait for the flying bots to come in and throw and EMP to destroy them all, now backwards roll AR and get shooting the HMG into the Bogey and go back into cover, keep doing this while keeping and eye out for any remaining enemies, you can use cigarettes here (they are useful) if you get hit and go into AR the overheat can mess you up against the Bogey but there is a way to survive this, the moment you get hit go to back left of the map and use that cover instead and hide until you recover and do the same method as before but using this cover and the Bogey will be easy, if you get overheat again just move back to the first cover. Kill the last enemy and run to the right side of the map, there is some rubble for Wave 3
Wave3: From here throw a grenade up to kill all the spawning robots, don't worry if you miss it because they all seem to run up the stairs so you can throw another grenade over the rubble and still get them all, it will just make your time longer. Take a LFE shot at the fire guy and shoot his tanks then using your Assault take the head off the non-drill guy (there should only be 3 of them left, 2 drills and some other dude) this is the 2nd easiest wave and is a good opportunity to relax if your not going for a fast time. No preperations needed for the next round (get some ammo if you are under about 50% of both your HMG and Assault to be safe), Go to the start stairs again :)
Wave4: LFE the spawning balls and go to the right side, AR early and shoot the turret guy in the face, roll to the middle and do the same to the other turret (keep rolling :D) and then throw down an EMP and just waste your LFE on the blue dudes (you don't need to worry about running out since you only need 3-4 shots in the next wave and there is an ammo for it at the start), you most likely wont get them all since they are spread out by now but you might as well, you could pick up a shotgun to speed this round up but there is some risk shotgunning the big guys, just roll and shoot them with whatever (really doesn't matter) keep one at a little bit of health for some preparation for the next round...
Preperation for Wave5: Pick up some EMPs (you probably wont need them) the Laser thingy (forgot the name) that locks on to enemies, get the Rockets and refill your LFE, finish the enemy off with the LFE (you don't need all the ammo anyway)
Wave5: Honestly this wave wasn't too bad, I found wave 2 more difficult because you have some leftover enemies so you don't have freedom to move, if you keep rolling in this wave its easy. Go near the middle and roll AR some LFE shots into the ball spawns (don't worry about accuracy you don't need the ammo and as long as you keep rolling Bogeys will not likely hit you) Ok so here we go, 2 Bogeys, no difficulty really, I kept rolling and they didn't hit me ONCE, and if they do just EMP the floor and go to either of the covers used in Wave 2 (if they aren't destroyed, if they are use the stairs themselves), simply roll AR lock on your laser thingy and keep doing that over and over, when out of ammo pick up the upgrade OR laser ammo in the middle (I'd suggest upgrade first as you get more total ammo), when they stay still to launch a big attack use the rocket instead as that extra damage is useful and they will definitely hit when they are more still, it doesn't matter which one you kill first since once one is dead the 2nd one is much easier and a bigger room for error that you could use almost ANY weapon to kill it, but it is worth mentioning the blue one drops some rocket ammo that could be useful.
It's really not difficult and if you follow this you should do it no problems, if you want a faster time I suggest switching your LFE for an Assault or HMG so after you fire a rocket you can pump out some shots to deal big damage, yeah it's higher risk but what do you expect if you want a better time
My final time was: 9m 43s, it's not fantastic but it's better than average so I'm satisfied.