Instructions
Battle the creature, reducing its HP by as much as possible.
Encourage the monster to use up its SP. The best way to do this depends on the creature's special moves. If it has an attack that hits multiple squares, for instance, you can position your fighters close together so the AI chooses the multi-square attack. If it has a long-range special move, keep your fighters just out of melee range.
Pick up the creature with one of your own characters, such as a Thief. Heal up your party, then have your strongest and toughest attackers retreat into the base.
Throw the creature onto your base panel. When you do this, any remaining SP the creature has will be converted into damage and unleashed on random party members. After that, the monster and your army will fight it out until one side is wiped out.