So, last night i just get my unstoppable achivement, i have some tips to help you guys getting through all of the 14 waves you need to (you just need to reach wave 15, not to complete it, don't misunderstand the achievement description) they're all possible alone, but i still recommand you to get a partner to do those.
You will start with a pistol (it will change depending on wich survival tier you are, but in the last tier you will start with a m16 red dot and a five seven), for the 5 first wave, use your knife to kill the enemies, 1 stab to kill, at your kill, be sure to take the mp5 on the ground, you'll need it later, use your pistol only if your going to die! don't waste your ammo without a good reason. at the wave 6 you'll have to kill 2 chopper.Take a predator missile and just kill those, if you are on a map that havea roof on the air support armory stay there and you'll get the achievement to destroy 2 chopper without getting hit, after wave 6 start using your mp5, and swap your pistol for an ak-47, your pistol should be empty. Most your going through wave, the enemies will give you more money but they're more hard to kill. Continue on your way to wave 15, at the wave 11 you'll have to kill a juggernaut, use 5 C4 to kill him, one at the time, wait for him to get back on his feet before throwing an other one. if you have extra money don't be afraid to purhase a self revive, those are very useful when you'll get downed and your teamate is in big trouble. NOTE: it take about 15-20 minutes to reach wave 15. Later on wave 11, you will have to kill 2 juggernaut, and you will have to kill infantries too, so use a lot of predator missile to kill the juggernaut when they're dropping of the chopper. NOTE: the chopper that is dropping the juggernaut can't be destroy, don't waste a predator on it. Wait after the choper that firing at you will come
camping spot:
not needed until wave 6, but look for it across your wave. Find a place with a roof, near of the air support armory, with 2 door or 2 way to come in, so each player can watch his side to avoid getting kill by behind.
Swap weapon:
easy: take mp5, pistol > ak-47
regular: take mp5, pistol > ak-47
hard: take mp5, pistol > ak-47, mp5 > acr 6.8
insane: pistol > mp5, m16 > ak-47, mp5 > acr 6.8, ak-47 > Fad
NOTE:fill up your ammo(750$) between wave, if you're low.
starting equipment:
easy: frag grenade, flashbang, self revive, 250 armor
regular: flashbang, self revive, 250 armor
hard: self revive, 250 armor
insane: 250 armor
NOTE: not sure i'll edit it on tuesday
AND BIG NOTE: don't quit after reach wave 15, the game won't save your progress, let the enemies kill you or just throw c4 and down yourself, then just hit B to quit to the menu.
Good luck