Total War: THREE KINGDOMS

This achievement isn't incredibly hard, but you'll want to play very conservatively. Difficulty doesn't matter, but I wouldn't do legendary at all as you'll want to have a lot of backup saves just in case. Limit the number of enemies and fronts you'll be fighting on (really you should only have one war active at any time) and focus on building up your starting area in the early-mid game. Try to target factions already fighting another faction or get other neighboring factions involved in your wars as well, to hopefully divide their attention. If you can expand through diplomacy, do so (remember that vassal and I think ally territories count toward the victory condition). If you have to capture a territory for whatever reason but don't think you can actually hold it, try to sell it off to another faction. If your territory does get attacked but you don't think you can win at all, do note that you can offer surrender and that does NOT count as a loss. Only if you actually fight the battle (either manually or through auto-resolve) does it then affect the achievement. Move your armies in steps so that you can back them out of a fight if you need to. If you're dealing with a particularly large army group or tough settlement, you can always bring in more armies and build your numbers advantage.

You can do this with any faction, but my personal recommendation would be Cao Cao (honestly, Cao Cao would be my go to for most of these achievements), because he can instigate proxy wars and his army-based schemes can really slow down and weaken enemy armies, or strengthen your own. I did Sun Jian, and his mercenaries are kind of pricey and about as strong as their regular counterparts, but they muster immediately, which makes them really nice if you need to raise a defensive force quickly. The mercenary captain retinues require a mercenary outpost in the commandery you're raising them in, but normal generals can also recruit the mercenary units themselves provided they're a high enough rank. Gongsun Zan's unique building (and maybe another faction's too, I don't remember) also lowers mustering times for all armies in its commandery for a similar effect. The only thing I would avoid are any factions focused on aggressive play, like Dong Zhuo and Zheng Jiang.

Edit: Just remembered that to have the option to offer surrender, there needs to be a general present at the commandery being attacked, either as an administrator or leading an army garrisoning inside the county. Also, there is a chance that the enemy will refuse your surrender, which increases the more likely they are to win. So even if you plan on surrendering that territory, you may still want to raise a garrison or assign an administrator with a full retinue to balance out the odds as much as possible. More likely you'll want/need to reload regardless, but the option is always there.