Later on for use monster decks you'll get two neutral cards with same effect: "if close combat is over 10 in power, destroy the highest value card". Great combo is to use the frost, which drops everything to 1/2 and if they've enough cards (which monster decks will) they'll have over 10 power (especially if they've a hero card which won't drop to 10) and boom you wipe out their entire row.
It's like defecating to the sunrise: Downright glorious.
But yeah, get decoys, lets you re-use the medic cards if they don't have spies, or lets you re-use their spies to build up your hand. This is a bit pointless for monster decks (since you don't want to be drawing more than one of each type into your hand)
As for building out other decks, make sure to check every innkeep for cards, and I think a couple of general merchants too. If you do the Gwent quests you should get some good cards from it (with the final leader cards coming from the tournament at the end as I recently found out).
Oh other advice: to protect against Scorch play a relatively high powered card, or only play cards lower than their hand (since Scorch is a two way street). Some of the monster swarms are built to do this automatically (aracheneas puts out a bunch of 4 in front row, then a 6 in the back, and the vampires have a shit ton of 4's then a single 5. But that's easy wiped out with the above technique)
I love there's just as much mechanics to discuss in the mini-game as there is in the main game
I've got a gwent module for Tabletop Sim if anyone would like to hit me up to try it out? (I've not loaded the game up at all so far mind).