Gwent.... It's about playing as few cards as possible. First thing is to know to throw a round when you are beaten. Know when the other guy is trying to trick you into playing just one more card. If they come in high, play your low ranked 'plain' cards.
Early game, get the Foltest leader card from the innkeep in white orchard to cancel weather effects. Keep buying every card and playing every game. Early on, play Northern Realms with Foltest and a bunch of Blue Stripes Commando. Any healer card, or commanders horn is good++ as well. When you get to the city, it's about time to switch to Nilfgaard. Spies, spies and more spies. The only weakness is against monsters, who have mega, mega infantry with tonnes of reinforcements. Keep a scorch handy for them.