This solution is similar to King fiShiZ's solution for solo survivor. Credits to him for this idea.
Take Warthog, map Crossroads, pilot Samson or Hawk (if you have low level), units: T99, Ratchet, Bucky, Generator (important), Devastator (if it is possible, if not, stick to T99s or Goliath).
Place 3 T99s at each bridge and one Ratchet behind them. Start at NW bridge, then NE, SW and finally SE. Enemies take one bridge at one time during first four waves. In the meantime destroy every fence near your base so bridge T99s will be able to support valley T99s in next waves. One player should transport soldiers to outposts to make things quicker. After this put 4 more T99s at each bridge (so you have 6-7 in total) and start making Generators (8). Put them in sockets (remember about outposts), it will give you additional 40 upkeep for units (or you can make even better deal -> one Generator allows you to make one more Devastator, which is extremely helpful.
Start making creeps during wave 5. Place 2 T99s in each chokepoint and place one Ratchet behind them. Try to place Ratchet closer to your base, so it will not be hit by enemies going for bridges and be able to repair T99s in valley. Your goal is to have 4 T99s in each chokepoint. Build upgrades for your AirMech in the meantime.
Final wave (9):
You should have 169 upkeep (129 + 40 from generators) and 136 storage space used. Place 6 Devastators (each valley and two bridges) in front of T99s and place 3 Buckys behind one bridge without Devastator to reach max upkeep. If you don't have Devastators, then make Goliaths or simply produce more T99s and put them in valleys (5-6). Ignore outposts if you lose them, repair Devastator if it has low HP. support your troops with your Airmech (this is when your Warthog will be really helpful). When you'll have 20 units left to kill pay attention because enemy will send Devastators. Produce T99s and put them in your weakpoints. You should pay attention to valleys, as Devastators usually get through those points. After bridge enemies are killed take your Devastators and put them in valleys. This should be more than enough to survive last wave and grant you achievement.
Note: You can use this solution for both Solo Survivor and The Four Horsemen and it should do the job.