Engaging enemies in Survival Mode is certainly different from those you'd find playing against humans. What tactics work well for Survival Mode which may not otherwise work well in multiplayer?
Survival mode never ends. I have found the best strategy thus far is pick an area you can defend easily next to 1-2 buy points, preferably ammo and grenades crates.
It needs to be an area with overhead cover to avoid choppers, ideally should only have 2 entrances at most. I have yet to find a good spot on maps that have 1 entry point.
Get the sentry granade launcher, have your partner do the same, Get Riot Squad reinforcements,
The idea is you can stay behind the sentry guns and riot troops and kill everything, You have Sentries guarding 1 entrance and the riot troops at another.
You will need explosives later on to take out juggernauts, normal enemies will start to take a whole clip to kill around lvl 30 if you dont got for headshots.
C4 and Claymores are helpful but can get a little expensive to buy every round.
Flashbangs are invaluable for taking the juggernauts out. By mid lvl 30's you facing 4 juggernauts, and exploding dogs with armor on most maps. It can get crazy. I have not found any strategy of displacing to be effective, the enemy troops will claymore most doorways and choke points.
Perks - Longer Sprint was the most helpful I found.
Equipment is more effective. Enemies almost never avoid claymores and other equipment, so using them usually gives you advance warning and easy kills. Enemies are dumb. They will come from the same direction in a straight line and you can pick them off easily. Camping works (mostly)...they won't 'remember' where you are and seek you out next time to kill you. Enemies do not work together, so their flanking is terrible. Usually you will have time to kill the enemies in one direction and then turn around and get them in another. All in all, effective use of midrange weapons (assault rifles, lmgs, some smgs), well-placed equipment, careful choosing of camping and movement, and use of available sight lines should help out.
I'm going to lay down some hardcore raw survival methods I used.
Map and camp sites:
You should have two sentries, preferably machine guns because the frags are slow and smoky; their power isn't all that great in my opinion. Make sure the sentries are under a roof at all times to prevent copters from killing it. Leave the sentries to fight juggernauts.
The L86 LSW, the fastest reloading LMG, is a must. Buy it with GRIP after the FAD appears on the ground. Take the M60E4, Red Dot Sight, GRIP as secondary if you get 30+ rounds.
Self Revive (always) and Body Armor is low priority, but should be put on after wave 17. Riot Squad or Delta shields once they're dead. (Delta is bad against juggernaut, and the riot squad drops riot shields once they're dead.)
Predator Missile on hand after sentries deployed, perk dropped down and riot or delta squad down because space on the killstreak is limited to one. Take the Extreme Conditioning or Sleight of Hand perks. You should order two at the same time so your buddy knows he is getting the perk.
Flashbangs are good because they stop juggernauts from shooting. Don't use RPGs the firing part is difficult because of riot shield juggernauts. It takes two RPG shots to take the AH-6 copter down. Use the Predator Missile to take out the copter and the juggernauts. If you do well, you can take out all the juggernauts in a wave by running around, grouping them, and launching one missile. If you can't, then this would be time for a Precision Air Strike.
Go for Headshot or Rampage or Knife Streak challenges early-game and then Kill Streak end-game when you have cover, riot squad, and sentries. You can get Flash Kill by using a flashbang and killing enemies while flashed -- but don't ever do this mid-game. Quad Kill means to use something (like a grenade) to kill at least four enemies simultaneously. I have done this, it's best with C4, don't do after mid-game because your riot squad and sentries mess it up.
Riot Shield's melee is equal to how much damage the knife does, but it will not count to the Knife Streak. The Riot Shield's face guard breaks in survival when held by a player. You will get more money if your accuracy is higher than 25% and no money if it is less than 24% so shoot a guy every wave. Refilling ammo and flasbangs will always cost $750. Arkaden is my favorite map because of the behind the counter, the two flanks converge and the roof over your head.