Rock of Ages

Start up a local war game with 2 controllers. Now build many units with your 2nd controller near the top. Then crush all these + all extra houses with people running around, with your main profile. This should get you around 5000 on round one. Repeat until you go over 7000 for the achievement.

I had the easiest time scoring this achievement in the story at Thermopylae (The first "real" stage after the two tutorial stages).

You'll start with $1,700, and will need to have $7,000 ON HAND to earn the achievement. This basically means that you can't build ANYTHING. Instead, you'll need to knock down buildings and run over people to earn cash, while at the same time moving faster than your opponent to beat down the door.

The good news is that you seem to get a 20-30 second head start, so you shouldn't have any issue getting enough kills to earn enough cash. Just run through the level, smashing towers, crushing elephants, and knocking over buildings to build up as much cash as quickly as possible.

Additionally, each smash on the door will land you about $1,000 (depending on how hard you hit it). As soon as you have $7,000 on hand the achievement will pop.

IDLE STRATEGY

2 controllers local split screen on Rome

start with 2.2k ea
press RT to bring up build menu, build the $ cash money sign. should be able to build 5 with your main account. now AFK till 5k-6k and add more, then AFK till you get the achievement. both accounts will get it if you do this for both accounts. press A on both controllers if they arent plugged in or they will shut off first.

idle strategies are useful for those who xbone at the same time. this is not the fastest strategy, its a strategy you can do while playing on the xbone or computer... or while doing chores/work.

when you max out at 9k the $ you built will disappear.