First is a fully charged iPhone 4 to call authorities if possible.
Next a Beretta PX4 .40 with 180-grain Gold Dots, a Streamlight TLR-1S, and an extra magazine. 5.5 years of ownership, 5,000+ rounds, 5 shapes of JHP. 100% reliability in every way.
Nearby access to a Remington 870 with six shells of PDX1 (1oz slug, 3 pellets 00 buck) if that doesn't get the message across.
Slightly less nearby access to a Daniel Defense M4V1 with 8 loaded Pmags and 1 loaded Lancer L5AMW for 270 rounds total, if we really, really aren't having success communicating.