The easiest would probably be Ascension. Run in circles on the lander area next to Widow's Wine with a PaP'd RK5 with Blast Furnace or something, and a Thundergun for monkeys.

At launch, the game features three on-disc maps: "Voyage of Despair", which takes place on the RMS Titanic; "IX", which takes place in an Gladiatorial Arena in Roman Egypt; and "Blood of the Dead", a soft-remake of the Black Ops II map "Mob of the Dead", taking place once more in Alcatraz Federal Penitentiary.