Maybe this is common knowledge, but in case it isn't, I thought I'd share.
 
If you are having trouble getting under 30 seconds in the Pit, try running it with pistols for both weapons.  Using and assault rifle messes up your accuracy rating (even if you hit the target every time you fire, some of the bullets will miss).  I did manage to get 30.9 seconds once using an assault rifle and a sidearm, but besides that fluke I hadn't been able to get better than 34.5, and my accuracy was always 75% or less.  Then I tried it with two pistols, and with a couple of double kills, got an accuracy of 114% which gave a time bonus of nearly 6 seconds, so I made it in 28.7 seconds.  The only tricky bit is that you have fewer bullets in a pistol, so you may have to change up when you switch weapons.

I think I got the achievement using the UMP and ACR. The former is quicker with aiming and moving while aiming, and the latter is mad accurate.

Dual pistols is the way to go. Fast swap and accuracy time bonus.

Dual pistols ftw. Just memorize where each target is, there are some spots where the spawn changes.

Teach yourself how to line up one target behind another so you can get two targets with one bullet. Big accuracy bonus and faster time.

Oh yeah. If you hit everything with one shot and taking out more than one target with a single shot you could end up with over 100% accuracy.

I went with the M9 all the way through. I ended up with a time of 34.7 seconds. It's JUST under the line, but hey, I still got the achievement.

I just stuck with the standard m4a1 using only one shot for each target. If you do that you never have to waste time reloading. Got the achievement no prob.

I did it with the M9 and the Desert Eagle. I used the M9 until it ran outta ammo, then switched to the Desert Eagle, making sure to hit two targets at once both in the first floor of the indoor area and the second floor too. I found that the trick is to reload while walking off (not sprinting) the ledge when he tells you to jump off. That way, your gun (desert eagle for me) will be pretty much done reloading when you hit the ground.

i rolled with 2 pistols when i completed it. aiming was pretty standard and i just memorized where the bad guys were. its a tough achievement but a good one.

i did it on PS3 on my first try. I used M9 / ACR (i think) 

PC it's much harder, because aiming takes more finesse. I got ~26 with the m9/knife and the desert eagle. The m9's knife is very fast, and great for hitting the guy on the way up the stairs.

Can I only get this achievement when I'm doing the Pit in the campaign, or in the spec ops mission?

Did it with a MP5K (or was that UMP) and USP .45. It didn't take me a whole lot of time to get through it and complete the achievement. Definitely much more easier than the time requirement for FNG mockup.

MP5k and the USP just as Meteora said.
 
It's the best way to do it in my opinion.