Serious Sam: The Random Encounter (2011)

Serious Sam: The Random Encounter plays a mixture of an action role-playing game and a Japanese role-playing game. During the open world sequences, the player may roam around freely, but can eventually hit on a randomized mob wave of enemies. These mob waves consist of ridiculously high numbers of enemies of all kinds, where the player is given the task to gun these down. (wikipedia.org)