Cards with the rank 5 are worth 10 points, and cards with the rank 6 are worth 11 points - making them the most valuable cards point-wise. You want to win the trick if any of these cards are present!
There are 14 trump cards in the game. This means that the gold 14 card is the highest ranked card in the game, and will always win the trick when it is played.
If you can, try to keep track of the higher ranked trump cards that have been played. This will help you make better decisions for which cards to play later on in a round.
When deciding whether or not to take the Boon, a good rule of thumb is to take it if you have half or more trump cards in your hand. Having more trump cards in your hand increases your chances of winning tricks.
If you have these cards in your hand:
Then you would want to take the Boon since most of your cards are trump cards.
But if you have these cards in your hand:
Then you would not want to take the Boon since only 2 of your hand's 7 cards are trump cards. Having less trump cards in your hand makes it harder to win tricks, which means your score will be lower at the end of the round.
An extra incentive to take the Boon if you have lots of trump cards is that if all players pass the game goes into Leaster mode. In Leaster mode you want the least amount of points at the end of the round, so having lots of trump cards in your hand will make it harder to win.
Assuming you have lots of trump cards in your hand, it's a good idea to lead a trick with a trump card. Since other plays must follow suit, they must play a trump card from their hand.
By forcing players to follow suit, you draw trump out of the game. In other words, for every one trump card you play you have the potential to make your opponents play 3 trump cards altogether (for a 4 player game). If you can exhaust other players' trump cards, it makes it easy for you to win tricks with lower trump cards.
In general, if you're on the defensive team you want to lead tricks with low value, non-trump cards. Cards ranked 1-3 make for great leads. This is especially true for the first tricks earlier on in a round.
Leading a trick with a high point value card (like a 5 or a 6) leaves an opportunity for the solo player to take the trick with a trump card.