Initialise attackpower first by
enemy1->setAttackPower(29);
enemy2->setAttackPower(99);
Then print them,
enemy1->attack();
enemy2->attack();
The main
would be like
int main()
{
Ninja n;
Monster m;
Enemy *enemy1= &n;
Enemy *enemy2 = &m;
enemy1->setAttackPower(29); // set the attack power first
enemy2->setAttackPower(99);
enemy1->attack(); // then print it otherwise junk values will be printed
enemy2->attack();
return 0;
}