Risk: Factions

Note that for this achievement to unlock your defending country must have one army and the attacking country must have at least SIX (because one army stays behind). You could wait for this to unlock randomly, but if you want to force it, there are several powerful ways to improve your odds:
- Get a bonus defense die from a victory condition. (Note that the rewards for victory conditions are assigned randomly at the start of each game, so restart until you see this one.)
- Get a bonus defense die from controlling three Barracks and being in a country next to the Missile. (Does NOT stack with the above defense die.)
- Put an airport in or beside your defending country. (This is another victory condition reward.)
- Use a second controller so your attacker is human, and have him explicitly attack with 1 die at a time instead of 3.

To help with planning, these are the odds of defending 1 army vs. 6 under various conditions.
Normal: 0.97%
With bonus die: 6.71%
With airport: 5.46%
With attacker rolling 1 die at a time: 6.75%
With bonus die + airport: 25.02%
With bonus die + attacker rolling 1 die at a time: 23%
With airport + attacker rolling 1 die at a time: 19.65%
With all three: 47.35%

So it'll come pretty quickly if you have any two of the above conditions.

This achievement can sometimes be acquired playing normally if you're lucky. However for those who don't want to wait for it even while playing such a neat game, I have my method.

First set up a custom game. Make it factions, world conquest and manual. Select Five AI players and choose the limbo map. This map has the temple on the left and the missile on the right.

Second, just play the game normally and try to win as many objectives as possible. Particularly important is to get the defense die and airport power-ups. I'd recommend setting up your territories on the islands on the left or on black fort and aim to fortify your territories then get Von's Temple (ridiculously powerful) and fortify it. From that point on, the game should be a wrap. However, to get the achievement there is an endgame.

Third, push your opponents to the right of the map and take control of the missile. Refer to the two images for an example. Set it up such that you've boxed them in on a continent (by filling up the territories which are their escape). Once that is done, try to let one opponent take the continent.

At this point convert one of the territories on the continent which is in range of your missile. If it has more than 1 unit on it, use your starting maneuver to remove them. Then freeze the territory with the least troops on it and end your turn.

Your opponent will then have no option but to attack this lone territory and even with just 1 troop of soldiers, you'll be rolling two extra die with the missile and defense die. Furthermore, I didn't do this in the picture but you could have placed an airfield on a territory connected to that territory to give yourself a better chance.

The best part is that even if you don't get the achievement on your first try, you can rinse and repeat this third step.

If you look at the first photo, you'll realise that I managed to get all the objectives and thus all the powerups but I didn't get the airport late enough for me to use it on the territory that would have helped.

Overview of the Endgame: http://www.flickr.com/photos/9752209@N03/5032740018/
Close up: http://www.flickr.com/photos/9752209@N03/5032120397/

The other solutions over complicate how to get this acheivement, so after trying them I've devised a simpler way.

Yes your defending unit needs to be 1 unit only, and the attacking unit needs to be no less than 6. The achievement pops on surviving for 5 hits (knocking 5pts of the attackers score), so the attacker can have 10 or 100 - it is irrelevant.

To set up:
1) Use a 2 controller game on a large faction map, World Conflict, a 4 player normal game should be ok, probably while you mop up the custom matches (so clear the AI).
2) Get the defensive dice objective for the defending controller.(Important)
3) Get one unit to 50pts+ for the defender, and a the same with an attacker. Collect these from all over the map so no spare units are available, but really for both the higher the better.
4) With the defender controller (going for the achievo) go on an attacking spree leaving a long trail of your single units behind you and move on to the next and keep going on the same turn. 15 to 20 map areas of single units should be fine, more if you can!
5) With the attacking controller, use the same route and attack the freshly won areas. Always use the quick roll method, no need to watch the dice nor animations for speed.
6) Repeat steps 4 & 5, it could take 3 or 4 tries so use your drafting to top your units up. The achievement should pop, a lot quicker than the setup of the other solutions. I got this on the 3rd turn... :o)

Simple but affective, no science required!

Ps If you do get to win the Airport Objective perk, just dont place this on a tile you are going to attack/defend keep it adjacent, as more often than not it will be destroyed (i didn't use this on my go, but it may help speed up your chances)

There is no way really to boost this. What it comes down to is luck and also due to the amount of time put into the game. i.e the more you play the higher the probability you'll net this cheevo eventually. I got by playing online. It actually happens more often than you would think. Just play the game and don't worry about it, it'll pop eventually.