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Think of it like the children's game "War" crossed with a TCG (Hearthstone's probably closest).
A deck is exactly 15 cards (1 Boss, 2 different Mini-Bosses, 12 Attacker/Effect). Duplicates are allowed for Attacker/Effect cards, with no limit on the number of copies.
Each player starts with 5 maximum HP, 2 maximum TP. Maximum TP increases by 1 each round until it reaches 10. TP is restored to full each round.
Each round, each player draws 2 cards (or 3 if they had an empty hand). The maximum hand size is 5; cards will not be drawn if there is no room for them.
Each player may play any number of cards, so long as the total TP cost is less than their maximum TP for the round. After executing the effects of all cards, if a player has higher attack than the other, that player wins the round and their opponent loses 1 HP. Otherwise, the round is a draw.
After being played, cards are immediately shuffled back into the deck before drawing for the next round.
The game ends when a player reaches 0 HP, at which point that player loses.