//bullit hits enemy
//in act
removeTouching(Enemy.class);

//from world -- with reference
private Enemy enemy = new Enemy();
//adding
addObject(enemy, 100, 100);
//removing
removeObject(enemy);

//from world -- without reference
//remove all enemies
removeObjects(getObjects(Enemy.class));
//remove one, maybe have to check if there is an enemy first
removeObject(getObjects(Enemy.class).get(0));

//from the enemy class itself, for example when his life reached 0
getWorld().removeObject(this);