I don't remember the rules exactly, but there are monster cards and supporter cards. Each monster card has an attack, defense, hit point and speed stat.
Each turn, the first move goes to the player with the fastest monster. Each player gets to move monsters from four rows (back, middle, front, and frontline defense) and attack. There can only be as many actions as there are monsters on the field.
When a monster is attacked, their damage is calculated by the attack divided by the stat and rounded up to the nearest hit point (where a hit point is the defense divided by the number of hit points). When there are no hit points left, the monster dies.
Most attacks are melee, but supporter cards allow monsters to attack at range, hitting monsters in farther back rows.
The game continues until one side has no monsters left.