Once you have unlocked the unlimited ammo cheat, choose the minigun into your arsenal before a game and whilst using it in-game, hold B. This gives you a impenetrable shield that holds any power up you have at the time for as long as you have the sheild on. Because you have unlimited ammo, this lasts forever and there is only way to get through it and that would defeat this cheat and so I'm not saying. Apart from that though, you are invincible.