Commonly you see people use TP or Easter Egg grenades. I have tinkered around with them and had poor results. Airsoft built grenades like the AI tornado and VFC Mapleleaf are your best bet. I general, you use a kill radius and in these circumstances a sound grenade or stun grenade will be better since the noise will improve the chances of people recognizing a kill.
Make, sure they are authorized by the event host and land owner.