If you're having difficulty with finding Firefight partners, here's an easy way to get the 200K achievements solo. It will take roughly two hours per map to get 200K, with 10 or so lives to spare (so you should easily get it first go).
Choose Easy difficulty, enable the Grunt Birthday Party skull then playthrough as you would normally - headshotting grunts and shielded guys (shooting their hands first so their shields move out of the way). Using the plasma pistol to remove shields of brutes and SMG to kill the flying bugs.
When the Brute Chieftain with the hammer comes along, kill him and take his hammer. Keep this hammer for as long as you can!
Save it until a new wave of enemies come, then chain together a Killionare and get the Dreamcrusher medal (10 kills with hammer). Once you get the 10 kills with the hammer (no kills with anything else during this time, including grenades), get a quick kill with your pistol and repeat this process.
If you run out of hammer ammo, simply melee the enemies. Don't drop the hammer with 0 ammo because it will disappear.
Don't be scared AT ALL to run into a crowd of enemies as it is easy difficulty and extremely hard to die.
Follow this strategy and you will have all the Firefight achievements with minimal effort, plus you can afford to pause it.