On the first level, when you meet the captain and he gives you a pistol, go up to the point where you have ammo for your pistol. As soon as you have ammo for it, turn around and go back to the captain. Kill him with your pistol and then you will here Cortana say "What the hell do you think you are doing?! Attention Marines, the Master Chief has gone rampant, let's bring him down boys." Then, swarms of marines will come after you trying to kill you. You can use this to practise your skills if you wish as no matter how many marines you kill, they just keep coming.