The Witcher 3: Wild Hunt

Note: No DLC packs are necessary to complete this achievement. The guide below assumes no use of DLC cards, and was written before the DLC released.

This achievement is gained once you have won a round of gwent with your total strength over 187. Note that it is only a round; you do not need to win the game. Also, this is unmissable; you can play gwent (e.g. with innkeeps) even after the end of the game (free roam time).

Strength in gwent is how much each unit card is worth (it is basically a form of points), this base strength on each card can be modified with certain cards such as commander's horn. You total strength is the combination of the above.

Realistically, you will need to have, or very nearly have the Card Collector achievement. This would mean to are skilled at gwent, and have all, or most of the cards available.

Generally, it is useful to lose the first round, this often means the opponent just passes on the next, thus allowing you to play all of your cards without worrying about weather cards or scorch. This can be dependant upon the opponent faction, I've found Nilfgaard opponents pass more on the second round than monsters.

Because you don't need to actually win the game, this is much easier than you may immediately think, however it does require some good RNG with your hand.

Here is how I got to this strength with different decks:

Monsters:

Using this deck and method I achieved 198 total strength after a few attempt for the perfect hand. iAlecP said that removing the commander's horn and dandelion, and using Eredin, Leader of the Red Riders helps.

The strength to the monsters deck is that the Vampires, Crones, and Arachas can call each other in. This means you only need one of each in your hand. More would reduce your ability to reach 187 total strength. Also, because most of these are close combat you can use a horn (or Dandelion, they do not stack) to double all that strength. This allows you to get to high strength very quickly.

Leader - Eredin, Bringer of Death (you can use any leader card, but this allows you to play a couple of cards at the start, to increase the chance of your opponent passing on the next turn).
Total cards in deck: 26;
Number of unit cards: 24;
Special cards: 2;
Total unit card strength: 153; (This number doesn't account for horns).
Hero cards: 8;

Deck - 2x Commander's Horn, Geralt, Cirilla, Yennefer, Villentretenmerth, Dandelion, Mysterious Elf (Avallac'h), Draug, Imlerith, Leshen, Kayran, Erath Elemental, Fire Elemental, Arachas (all four), Crones (all three), Vampires (all five).

Ideally, your hand will contain: Dandelion (If not, then a horn), a Vampire, a Crone, an Arachas (only one of each, no more), Mysterious Elf (Avallac'h), then the rest could be anything, obviously it is preferable to have higher strength cards (Kayran is good for +1 to all melee cards). A commander's horn as well as dandelion is ok, you can use it on the siege units.

Round one - If you have Avallac'h then play him first, then place any non-hero card that isn't the vampire, crone, or arachas (e.g. Dandelion), play another card as well, then pass.
If you are lucky you will get two cards with are not vampire, crone, or arachas (if you already have one of each).

Round two - Hopefully, your opponent will pass immediately. Regardless, play all of your cards (start with hero cards if they continue). If you don't quite get there, don't worry, try again for the optimum hand.

Round three - you will probably lose this one, but it doesn't matter.

Nilfgaard:

Using this deck and method I achieved 203 total strength on my first try.

Nilfgaard's potential is in it's use of spies and its card's 'tight bond' ability which adds the units base value to the unit if another card of the same name enters the field (contrary to what the game says, they are additive not multiplicative).

Leader - Emhyr, the Relentless (allows you to take an extra card, or even retrieve a spy to gain two more cards).
Total cards in deck: 26;
Number of unit cards: 24;
Special cards: 2;
Total unit card strength: 171; (This number doesn't account for horns or tight bond).
Hero cards: 9;

Deck - 2x Commander's Horn, Geralt, Cirilla, Yennefer, Triss, Villentretenmerth, Dandelion, Mysterious Elf (Avallac'h), Letho, Menno, Morvran, Tibor, Black Infantry Archer (both), Heavy Z Fire Scorpian, Stefan Skellen, Shilard, Cahir, Young Emissary (both),
Vattier, Impera Brigade Guard (all four).

Ideally, your hand will contain dandelion, and all or most of your spies, then any combination (it shouldn't matter too much because you will gain up to 8 more cards from your spies, 10 if you retrieve one with your leader ability).

Round one - play all of your spies here. Hopefully you will get all four Impera Brigade Guard, and both Young Emissaries, however I got 203 with 3 Guards and one Emissary, so don't worry too much. Then pass.

Round two - Now you can go all in. Start with your hero cards to be safe if the opponent continues to play, then place dandelion and stack your tight bond units to get over 100 easily, then just systemically place your cards. If you don't reach >187 then just try again with better RNG.

Round three - doesn't matter, you have the achievement.

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I can add a guide for Scoia'tael and/or Northern Realms if anyone really wants them, but for the sake of reducing the volume of text I will leave them out. If you are struggling make sure you have tries the Nilfgaard deck and method, as it is less reliant on RNG.

RNG means random number generator, and refers to the way the computer randomly determines which cards you receive.

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Thanks to Lyzyrd for pointing out the lack of necessity of DLC.

Here is a video showling a easy way to get the achievement.

I used Northern Realm deck vs a Northern Realm deck to get as many spies played and for the NPC to quit on the first round