Ive never read any real strategy guide on it, but the basics are pretty easy. Protect your vulnerable sides while attacking those of your opponent. Pay attention to the rules in play, as they can drastically shift things. And in general, i personally seem to win more when i play offense, and draw more when i play defense. What i did when i was starting out was play all the weak NPCs i could to get my deck started. Generally, the more it costs to play an NPC the better they are. So you can use that as a guidepost to see what to expect. In general, each card has at least 1 weak side(5 or lower), so what i did was seek out cards with 6s or 7s facing each direction to build my first deck - replacing things as i got better versions. The more cards you collect, you get achievements that let you put stronger cards into your decks(up to a deck of 4 three star cards and 1 open), making it easier to tackle those harder NPCs. You can also build different decks to use with different rule sets. For instance, there are plenty of low-star beastman cards allowing for an easy ascension deck. And low level cards often have multiple weak sides, letting you build a superior reverse deck early on. Look for NPCs with those rules, or watch the regional rules that can change daily and use these to your advantage to beat harder NPCs before you would otherwise be able. Otherwise, you could always just get really good at baiting your opponents into making bad plays, and beat them without upgrading your deck at all.