Catan

Easy Mode
4 AI
10 Victory Points - Standard Game

To get the Largest Army card, you need to use at least 3 soldier cards. To keep the Largest Army card, you need to have used more soldier cards than anyone else. To get a soldier card, you must build development cards. There is a better chance of getting a soldier card than any other card, by about 50-60%, estimated.

After 10 games of doing this, you should get the achievement.

TIP: Do NOT do this achievement along with Citizen and Builder of Catan. If you are to get all 3 achievements, or any other achievements, simultaneously in this game, it will record one and void all the others unless you load Catan onto another Xbox 360 and recover your gamertag. This is the only known workaround. View Xebu's comment below for more information.

EDIT: Win or lose, as long as you end the game in possession of any card, it will count towards your achievement.

Aerodynamo's solutions absolutely works, but I thought I would add another detail to potentially spare some people extra time.

The achievement says "End any game..." so I thought I could just play 3-player with easy AI to speed things up. Turns out that 10 3-player games don't work for this (found out the hard way). I only got the achievement after 10 4-player games w/ Largest Army.

Just thought I would put that out there so people don't end up wasting time trying to short cut like I did.

Aerodynamo's solutions absolutely works, but I thought I would add another detail to potentially spare some people extra time.

The achievement says "End any game..." so I thought I could just play 3-player with easy AI to speed things up. Turns out that 10 3-player games don't work for this (found out the hard way). I only got the achievement after 10 4-player games w/ Largest Army.

Just thought I would put that out there so people don't end up wasting time trying to short cut like I did.

Aerodynamo's solution works perfectly well
However, using my setup you can do this in half the time.
On single player, set up a lobby and press the (Y) Button for setup options. Here, you can change the settings to the ones below!

My Setup:
- 4 Easy AI (cannot be 3 AI, higher difficulty will be sufficient)
- Fast Setup
- Normal Victory
- Fixed Positions (Big time saver)
- Friendly Whatevers (Your Preferences)

What you should know
- Dice Rolls of 7 will not count towards largest army
- to get a soldier card, you have to buy a development card (1 Wheat, 1 Ore, and 1 Sheep)

You will be placed in heaven resource wise.
1) You will start out with 2 Bricks, 2 Ores, and 2 Sheeps, and your 2 settlements will be 3 spaces away from each other.

2) Trade 1 ore for 2 wood (should be easy with AI on easy)

3) Build 2 roads trying to connect the towns. Trade one of the sheeps for a wheat, and buy a resource card. This is the beginning of the strategy.

4) Trade excess resources and focus trying to get development cards.

5) After you get your 3 soldier cards, build a settlement at the ore and wheat closest to the shore (only acceptable place) and then focus on trying to make them all cities or collect victory point cards. C: good luck, Hopefully this helps someone!