Aim to injure. not kill[edit]
Whenever possible, try to reduce your opponents cards down to as close to 100 BP as possible. When you kill an enemy character card, you give your opponent the opportunity to replace him or her with a more powerful character with more BP, and to gain more SP that the card grants. By knocking a character card down to 100 or 200 BP, your opponent is stuck with a very weak fighter who won't be able to attack you for much damage, or block a united attack for very much benefit.
Two cards is ideal[edit]
At the end of every turn, you should aim to have only two card slots filled, not three. Whenever you have an empty card slot, it's an opportunity to add another character and gain more SP. When you start off with one empty card slot, fill it with a new character card, and then ideally try to attack with the remaining cards in a united attack. Choose the character with the lower BP first, and the higher BP second. This should get blocked by an opponent's character, who is hopefully strong than your weaker character. The weaker character will be killed, the opponent's character should be killed, and your remaining fighter will damage the opponent, leaving a space for you to play a new character card on your next turn.
The Substitute action card[edit]
Characters like Iori, Genjuro, and Ukyo have high amounts of BP, but come with debilitating Abilities just as an inability to counter attack or immediate death. The Substitute action card can render these characters more usable by swapping their Abilities with an opponent character's abilities (which also has the effect of making one of their character cards less useful.) Consider packing a Substitute card if you include any of those characters in your deck.
Guile's ability[edit]
Similar to the strategy above, Guile's ability "Somersault Kick" can benefit you when you want to play a character that has a negavtive Triangle ability. For example, if Guile is on the table (whether he belongs to you or your opponent, it does not matter), Blodia's 10 SP cost to play is negated. You would not be required to return a card to your hand if you played Ring, or discard a card from your hand if you played Setsuna if Guile is present. Note that this effect can also hurt you, as it cancels positive abilities such as Akuma's Shun-Goku-Satsu instant kill ability, or Nakoruru's Balm of Nature healing ability, so be careful who you play when Guile is on the table.
Powerful ability characters[edit]
There are several rare high ranking cards that have incredible abilities to eliminate characters from your opponent's side of the table. Akuma can eliminate any character you choose. Wild Iori can eliminate any character whose BP is 800 or greater. Evil Ryu can instantly eliminate one character in Freeze Mode when he is played. Yamazaki can't instantly kill anyone, but he can put an opponent's card into Freeze Mode, which prevents them from blocking your attack.
Yashiro can instantly return an opponent's card that is out on the table to their hand. While this isn't as good as eliminating them, it can make it harder for your opponent to block the next attack. Just be careful as your opponent is given a second opportunity to play that card, including any Triangle ability and the SP they provide, and they get all of their BP back.
Sometimes it's better to not defend[edit]
Although you may want to block every attack that comes your way, sometimes it is better to let the attack go through so your opponent's cards are stuck in Freeze mode, and will be unable to block your next attack. Consider this before blocking, and make sure that you're not passing up an opportunity to do more damage to your opponent than they would do to you by not blocking their attack. It's a risky strategy that can really pay off under the right circumstances.