Utilize the face cards! The face cards have special effects on them that amplify the power of your regular suite cards. Knowing when to play these cards can be a real detriment to whatever strategy your opponent is cooking up, so here's a quick reference list of what each effect does.

Queen
Switch - Targets opponent's cards. Switches the targeted card's suit to the other color.
Poison - Targets opponent's Hero. Inflicts poison, which causes them to take 1 damage every turn (which includes turns during the card drawing phase).

King
Pierce - Targets your red suited cards. Adds piercing effect to target card, allowing it to go through armor. Useful if your opponent likes to stack armor!
Absorb - Targets opponent's red suited cards. If the targeted card is played during war phase, the effect is nullified and you gain mana equal to the what the card played was worth.

Ace
Possess - Targets opponent's black suited cards. If the targeted card is played, the armor is instead granted to you.
Leach - Targets opponent's Hero. Absorb a little bit of their health!

Joker
Immune - Targets any of your suite cards. Protects the targeted card from face card effects.
Reflect - Targets opponent's red suited cards. If the targeted card is played, the damage is instead directed at your opponent.