This level is actually very easy to get platinum on when you use the right strategy. To start off, it doesn't really matter where you spawn since this is a capture the bag match, and all the points are in the same general location. So, wherever you start out, head down the street and stick to the left side, where you should see a cloaking powerup. Pick this up quickly, and proceed further until you see a small room with body armor a little bit farther up.Run in and get the body armor, and then continue through the room until you are back on the street again. It is a small shortcut but it helps, and plus you get the added protection.

Continue down the left side of the street and you should see a door at the very end. Head through and turn left and pickup the speed powerup. Immediately head back out and go down the street towards the enemy bag. Strafe a little to avoid being hit and run down until you see the end of the street with the bag. Pick up the minigun right next to it and then pick up the bag itself. Hit the secondary fire to keep your minigun warmed up and then run back towards your base. Since you are cloaked and have the speed PU, it should be relatively easy to get back to your base quickly and w/o being hit. You can score in about 30 seconds (first time) this way, and the speed PU should end right as you arrive.

Now, you are going to go the exact same route, except, you will not be picking up the speed or cloaking powerups because they will not have respawned yet. So, it will take a little longer. However, once you score the second time, they will have respawned and you can pick them up again. If someone takes the cloaking PU don't worry about it though because all you really need is the speed.

In my experience with the level, this never happened however. So basically you are going to keep up this two run cycle until you win. Nevertheless, there are a couple of things to watch out for.

One, they might pick up your bag and your teammates will not be able to secure it. This is not a problem as long as you are aware it has happened. They take the same route you do so just shoot them with the minigun when the come along and quickly pick up the bag.
Secondly, you do not want to die. If this happens you should probably restart because it is near impossible to win. Luckily this should not occur because with the 2 body armors and the one health pickup along your route you can take a lot of damage, and kill everyone else along the way. I have only died once on this level and that was from a (un)lucky grenade shot. So just repeat the run five times and you should get platinum for this level, which I believe is 5 in under 3:00. I got it on my third try in 2:51 using this method.
One last note, if you find you are 5 or 6 seconds over your first few tries, like I was, try taking a more direct route by not strafing, not going for the health and body armor if you don't need it, and securing the bag quicker. Good luck!