The Against All Odds achievement is one of those which you’re likely to obtain by playing through a normal campaign. If you play for long enough, to the point where you’re beginning to draw the attention of army commanders from some of the largest factions, there’s a good chance that you’ll end up in a conflict where you’re massively outnumbered. You can of course make this happen much sooner, but you’ll need to have at least a few hundred high level troops, and you should be confident enough in your own combat abilities to make sure that starting a fight with a much larger force doesn’t look like suicide.

One of the best ways to obtain this achievement quickly is by fortifying one of your settlements, populating it with a decent garrison and a capable army, then attracting the attention of massive armies which are commanded by enemy leaders. You can do this by raiding their villages and caravans, or just by attacking some of their smaller armies. Eventually, the leaders will become angry enough to hunt you down and besiege your settlement(s). You’ll find it much easier to defeat a larger force than your own if you have the advantage of being within an adequately defended castle or fort, so try to hold up in one of these, and be sure to have a significant number of skilled archers to whittle down enemy numbers as they attack!