Let's refresh our memory of the basic principles of the game. It is worth noting that gwent in the book saga and in the game “The Witcher 3” are markedly different. While in the books gwint is described as a regular card game like poker, in the game it has been turned into a collectible card game, where the player's success strongly depends on the quality of the collected deck.

There are 5 main decks in the game with their own features:

Kingdoms of the North - the owner of this deck takes an extra card after each round won.
Nilfgaard - this deck wins in case of a tie on points.
Monsters - One random card from this deck remains on the table after each round. Squad cards from this deck also have a feature that allows you to choose all identical cards from the deck (not from your hand!) and play them at the same time.
Skoya'taeli - at the start of battle, they decide who moves first.
Skellige - This deck appeared in the Blood and Wine expansion and becomes available only in Toussaint.

The cards in the deck are divided into leader cards, hero cards, squad cards, weather cards (“Frost”, “Downpour”, “Darkness”, “Clear Sky”) and special cards (“Scarecrow”, “Execution”, “Commander's Horn”)

Squad cards are divided into 3 categories - melee squads, ranged squads, and siege squads. This affects a number of layouts and which weather card can be used against them.

Some squads and heroes have their own unique abilities - Medic, Spy, Rush of Strength, Strong Link.

At the start of the game, a player draws 10 random cards from the chosen deck and may replace 2 cards from his hand before the start of the round.

Both players take turns, and the round lasts until both players are willing to fold or both players run out of cards.

A game lasts until one player wins twice and consists of two or three rounds.

I recommend always playing with the Kingdoms of the North deck.

Using this deck and the tactics described below, you can go through the entire game and defeat any opponent with minimal replay.

And the tactic I'm proposing boils down to the following:

In the first round, we toss the opponent spies (if any), drain all weather cards in hand (if any) and use the leader ability “King of Temeria” (the default available leader card definitely has it). After that, we start putting the squad cards with the minimum strength. The goal is to force your opponent to waste their troop cards when making counter moves. Why waste? Because we are purposefully draining the first round and are not going to put more or less normal cards against the opponent. So after we run out of all weather cards and squad cards with 1-4 unit strength, we press “Pass” and let the AI win.

In the second round, the opponent, as the winner, moves first. We react to his actions by placing identical unit cards with the same strength, and if we have a choice, also of the same category that the AI is using (this will offset the risks of using the weather cards he currently has). Our goal is to win this round with a minimum advantage.

In the third round, we go with everything we have, laying out the remaining cards in our hand from the weakest to the stronger units (thus leveling the possible damage from the enemy's use of the “Execution” card). And then we win again.

Optional:

Sometimes an opponent may decide to spam in the first round with a minimal points advantage. In such a case, there is no point in draining the round. We win it and prepare for the next round by continuing the tactic of drawing good cards from the opponent.

What is the best way to deal with the “Scarecrow” card if it comes into your hand when dealt?

If you have to draw a squad card with a power of more than 4 in a flush round, you should take that card back before you flush.

Otherwise, you should keep the Scarecrow in your hand in case your opponent wants to toss us his spy, so that you can pick it up and toss it back in a flash

A very good option is to have a unit with the Medic ability in your hand in addition to the Scarecrow. Then in subsequent rounds you can pick up the exposed Medic and put him out again, drawing another strong card from the discard.

By the way, Medic cards, regardless of their strength, should be kept until the final round, so that you can get an additional advantage by drawing the strongest card from the discard.

You can alternate decks during the game for variety and interest, but if you want to play as efficiently as possible, focus on developing one deck and use only that deck.

Buy cards from merchants at the beginning of the game. This will give you a good head start against the first random opponents.

The stronger your deck, the easier it will be to win. Don't neglect to do additional quests related to gwent, which will give you additional cards for winning.

Don't hold on to weather cards for later. In some cases they can really give you an advantage, but it's much more efficient to dump them at the beginning of the game, forcing your opponent to spend squad cards in return.

Against a Monster deck, the Execution card is particularly effective.

All gwent games in The Witcher 3 can be replayed an infinite number of times until Geralt wins. The exception is the gwent tournament in the quest “High Stakes”, when you need to win three times in a row. You should start this quest with an already powerful deck, and do not forget to save right before the tournament, because the element of chance has not been canceled.

The tactics described above allow you to win in gwent with minimal replay, regardless of the deck your opponent uses.

The main thing is not to try to win in gwent in 2 rounds and not to be afraid to pass in order to gain an advantage.