Gwent high score round of 554, played on PS4 and max difficulty setting.
Round 1: Geralt wins passively (0 points wasted)
Round 2: Geralt wins playing all cards in the winning round (554 points)
Since it is a best-of-3, and Geralt won 2 rounds, there is no Round 3.
Question: why not Horn on front row? Answer: Dandelion/Jaskier card already applies Horn effect.
Question: did you lose Round 3? Answer: Geralt won both rounds and the game is over.
Question: how to get five Blue Stripe commandos? Initially only 3 were available. When patch v1.1 arrived for the first DLC, it was possible to get 5 for some time. About a year later, CDPR patched it back to 3 maximum. So it's no longer possible to get 5 Blue Stripes if your game has the latest patches.
The strategy I used is the same as my previous 465 point run using vanilla cards, except here I use some better cards only available in paid DLC. The theoretical max is 594 points if Geralt gets lucky enough to collect all "spies" on his side of the board. To get a very high score you must play on max difficulty because the opponent has more spies and decoys than lower difficulties, and the opponent having better cards actually help you get a higher score overall.
The basic idea of get very high scores is to play spies and decoys to resurrect cards, and play much more than the typical maximum of 10 cards (you can play up to 34 cards with Nilfgaard deck). You also want to win a round by wasting very few points, and then win a round by using all of your cards for a big score. The best opponent is Sasha in Passiflora in Novigrad since she has 4 spies, 3 decoys, and 4 medics, which keeps the round alive for a long time. She also has 3 scorch cards, but you can be clever about what cards you play and in what order to prevent her AI from using them.
Getting 500+ takes many attempts because you need a few things to work out just right:
1. craft a deck that is possible to extract fully. The Nilfgaard and Northern Realms decks can both extract 30+ cards using spies, decoys and medics.
2. the four Gaunter cards summons are worth big points, but you must draw Gaunter before any of his three Summons. For each summon you draw before Gaunter, you "lose" a card. It is just luck to draw Gaunter first. A safer tactic is to ignore these four cards and use Trebuchets, less reliant on luck but lowers the theoretical max score to 578.
3. need to manipulate the opponent AI into feeding you spies. Sasha has many spies and decoys, and can Steal one card. Use these for your own benefit let Sasha recycle spies so that you can use them yourself.
4. spies are worth points and you want them to be on your half of the board at the end. There are 8 spies in this match and six are worth points. In this video I only got 2/6 spies, which is not good. There are tricks to manipulating Sasha into feeding you spies, such as inviting her to Steal a spy from your discard pile, scorching her spies, and generally ensuring that she uses her Medic cards on spies and not anything else.
Note that although Sasha has 3 scorches, she has no bad Weather effect cards. You can avoid the scorches by playing low point cards first so that a scorch would hurt her. She will not play it. If you've already beaten the Tournament quest, it's still possible to score 490+ rounds against random end-game merchants who use Nilfgaard decks.