Tips regicide

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.

Solo tips:

1. If you see a lot of diamond cards in your opening hand, this indicates that there aren't as many remaining in your deck. Use your diamonds carefully; you may not see more of them for a while!
2. You want to go into each diamond opponent with a full hand, if possible, because you need enough damage in your hand to quickly defeat them, the discard fodder to take damage yourself, and the diamond cards to keep going afterward. The 8-card hand limit makes sculpting your hand in advance much easier to do than in multiplayer, so make use of this advantage!
3. Don't neglect your hearts! The thinner your deck gets, the harder it is to recover, as your heart cards will have a lower chance of putting back diamonds and power cards. Getting jumped by a heart opponent while your deck is thin can be backbreaking. Not even the Jester can save you if you run out of cards!
4. Hearts are worthwhile even when you only have a few cards in the discard pile. If you have anything at all in there that you want to see later, even if it's just one card, you'll want to put it back ASAP. For example, if you discarded the 5 of spades and the 5 of diamonds, and you want to see them together later in the game, then you need to recycle them while your discard pile is still small.
5. You might be tempted to fire the 10 of clubs immediately if you see it in your opening hand, but you will probably want to hold it for the Jack of diamonds. If you draw the 10 after the Jack of diamonds is already defeated, the Jack of spades is okay to use it on, since he can be annoying.