GWENT: The Witcher Card Game

This achievement is one of many that are luck of the draw, as is the case with most (if not all) card games.

To do this, you need to have several fortunate events happen at once:

1.) Your opponent needs to have played a Legendary unit. These are the Gold cards.

2.) The Legendary unit needs to either be weak or you need to weaken it enough that a Common unit can defeat it.

3.) You need to have a Common unit that can directly attack other units. These are the Brown cards.

4.) All of these need to be in play at the same time.

Now, acquiring Gold and Brown cards, and having an opponent do so, is the easy part. Even if you don't have access to them, you can play the Arena, which makes you pick a deck of randomly chosen cards, which is a solid mixture of Gold and Brown cards.

Actually pulling this off is a little more tricky, and largely comes down to luck. Ideally, an opponent needs to have placed a Gold card already, so all you need to do is take it down. Gold cards are typically the more powerful units, so you will most likely need to weaken it directly. A wide variety of card effects can all do this, as this is a main mechanic of the game.

You will also need at least one Brown card that can take it down. If you have a Brown card that can do 3 Damage on a Deploy, for example, then you need to focus on getting that Gold card down to 3 or less health.

Finally, the achievement description is worded a little poorly. You need to destroy a Gold unit with a Brown unit. Using spells or traps don't work. It has to be unit vs. unit to unlock this achievement.

My advice: Don't actively go for this achievement right away. You will get frustrated counting on luck. There are many opportunities to unlock this achievement while pursuing others. For example, completing an Arena run or trying to win a match with no Gold cards in your deck. If you are impatient and want to go for this now, however, follow the steps above.