This is a strategy the group I was with used and was ultimately successful with.
To win a game of swarm, you must last 15 minutes, and this is broken into 5 minute chunks. After each five minutes, an airstrike will kill every enemy on the map and give you a breather, and time to defuse your CP (which is often necessary). Be aware that you do not lose just by losing the CP. The enemy must hold the control point for a long time to make you lose (it's the same amount of time it takes to claim a CP in conquest, which I believe is 30 seconds), so the best strategy may sometimes be to just wait for the airstrike before going in to defuse.
In swarm mode, enemy shielding is never an issue -- even the most "advanced" enemies that attack you at the end of an insane swarm round have pathetic shielding. Armor is your problem by a long shot, and engineers, lieutenants, and captains have a ton of it. As such, plan your loadouts according to that -- use slug, rail, or especially incendiary rounds on your weapons whenever available, For the assault rifle, use the automation mod. Everybody should be packing napalm mortars and a repair tool.
As a general rule of thumb, use your napalm mortar often, especially when you see large groups of enemies. Their shields will be stripped, and they'll be taking fire damage for several seconds that chips away at their health. Napalm mortars are especially helpful when enemies are too close to your control point; dropping mortars directly on the CP will be an invaluable help near the end of the time chunks.
You will want to do this on Whiteout. Position two players on the catwalks on the near side of the excavator, and the other two will be camping further back, one behind the crates on the right (if you're looking at the excavator from the control point) and one behind the circular building to the left.
To begin, the first five minutes is all infantry and is the second easiest bit of swarm. The guys further back will want to thin out the enemy ranks as much as possible, as they neatly file over the hills on both sides of the excavators. Napalm mortars and assault rifles are best suited for this purpose. The two teammates on the excavator will be dealing with any enemies that remain. The only thing you will want to buy during this time is one supply depot, for obvious resupply reasons.
After the first airstrike comes the second and most difficult chunk of swarm. Spam *only* missile turrets between the two guys in the back as quickly as possible; do not bother with any machine gun turrets. Each person is allowed two turrets, so you should have eight of them back there. Everyone should also drop their supply depots within range of the turrets so they stay up as long as possible being autorepaired. After this point, it is imperative that as each person's deployables get destroyed, that they redeploy a new one to replace it as quick as possible. Vehicles on insane swarm have tons of health and will take everyone out in mere seconds if you all allow the turrets to simply be destroyed and never replaced. You may laugh uncontrollably at how fast the enemy vehicles get destroyed. Beyond that, the infantry is generally mangeable. It's impossible to not lose your CP during this segment, just try to hold out as long as possible so you can casually defuse it after the airstrike.
Between segments 2 and 3, you may opt for a pair of your teammates to call in mechs. Be aware that these mechs will be destroyed almost immediately if they venture beyond the turret picket, so they will want to be heavy fire support instead of melee machines. If used correctly by a pair of people who don't showboat, they are a massive help. Regardless of who grabs the mechs, you want to make sure that both remaining players on foot are in the back thinning out enemies with their mortars and assault rifles at extreme range.
Now comes the third and easiest part of swarm. There are vehicles like the last time, so do a quick deployable check in the couple of seconds you have to gather your wits. You will be fighting Spear and Crimson guards for approximately 2.5 minutes, then there will be a lull as nothing approaches for a good 30 seconds. Double check your deployables, and all you have to do now is prevent hacking of your control point for at least one minute and 45 seconds. After that, simply wait for the airstrike and defuse your CP.
To make a long story short -- missile turrets and napalm will save your asses. Make no mistake about it, without at least 4 missile turrets deployed at all times after the first airstrike, you will lose.