Don't rush the kills, you can play the game pretty slowly.

If a diamond unit can come next, try to think of what you can do against it, you maybe need to kill with diamond the previous unit.

Try to play spades when a new enemy comes.

Against a clubs enemy, a good strategy is to play spades to reduce their attack at 0. Then you can play heart, diamond, and low cards a lot. It can be done against hearts too, but is less powerful.

Don't be afraid to discard more than what is needed, if it's to discard a bad card like a 2. Especially if you hold some diamonds.

Usually when an enemy comes you can either try to kill them fast, usually with clubs, or to stall by playing spades. You have the option to try to recover too by playing diamonds (and heart), and let the next player decide what to do with the unit (but then stalling is less effective).

Keeping a low attack (<3) on an enemy is often better than having a 0 attack, except against hearts and when you need hearts.

When attack of the opponent is low, don't hesitate to yield, it lets you discard your bad cards.

Against a clubs enemy, don't hesitate to play clubs if you plan to stall against later enemies.

Advanced:

1. The cards played are put in the middle.
2. The cards discarded are put in the discard pile.
3. When a heart is played, some random cards from the discard pile are put in the bottom of the tavern.
4. When an enemy is defeated, the cards in the middle are put in the discard pile.

Knowing that:

1. if you start with a lot of diamond, there is a good chance you will lack of diamond in the tavern. That means if you discard too much diamonds, there is a great risk to get no diamonds for a while.
2. if there are few cards in the discard pile, you can try to put some cards there which are good together and play a heart just after. (For example, if you kill a jack enemy with heart and then play 5 of clubs and any other 5, you will kill the next jack with perfect damage, and your two 5s will be solo in the discard pile. If you can manage to play another heart just after, the two 5s will next to each other in the draw pile which is great, or, if your teammate just played two 5s, you have higher value of playing another 5 immediately, to have a greater chance to have the 5s near to each other on the deck.)

At 2 players, if you discard more than what is needed, that usually means you got some diamonds.