First, of course you must build up your gwent deck by buying cards from merchants and playing everyone you can.
I did this with a Northern Realms deck with the perk to clear weather effects. When building your deck, remove weather cards or any special cards that don't help your attack, such as scorch. I had 3x warhorns, cards that bond with each other to double their attack, three spies, and some medics. This is also easier with an opponent who uses spy cards, because you can revive them from the graveyard to get even more cards.
In round 1 you want to get a hand with all your spy cards. You don't need to start with all of them, but you want to draw them all in round 1. If you don't, forfeit and play again. The idea is to use your spy cards to get the biggest hand you can. This is a good idea in almost any gwent match! If you drew a medic, you can revive any spies the enemy used on you in round 2. If you built your deck with a small number of good cards, you should be able to get most of your cards in your hand. In particular, hope you get your war horns. By playing every card now, clearing bad weather, and using two or three warhorns, you might be able to hit 187 strength if your deck is good enough.