The general strategy for fighting 1v2 is to play defensively until you can use revenge and go on the offensive. If you're decent at the game then this shouldn't be a difficult achievement to get. Playing any 4v4 gametype will increase your chances of being in a 1v2 situation.
I highly recommend playing the basic and advanced tutorials, as they explain the basic mechanics of the game and specifically how to fight in uneven situations.
Some key tips to help you:
1. It's easier to stay locked onto one enemy. Switching back and forth can be confusing.
2. Blocking the locked-on enemy works the same as in a 1v1 situation (block according to their stance). However, blocking the non-locked enemy works completely different -- you only need to block in the direction of that enemy. So if the enemy is attacking from your right, you will see an attack indicator on screen and you can block by moving
to the right.
3. Revenge is key here. Successfully blocking attacks from multiple attackers will build your revenge meter quickly. Focus purely on defense until revenge is ready; then try to trigger it when an enemy is close by so that they get knocked back. This will let you get one or two free shots in and hopefully get a quick kill.
4. Look to take advantage of the environment. If one enemy is by a ledge, guard break
them and then guard break
them again to throw them off. This will instantly kill them and should grant the achievement.