This is single player game, So answer is simple = never. And thanks God for that. All attempts to add multiplier options to single player games causes harm to those games, because it screws balance and or wastes developer time. Like it was in Fallout Tactics (there are like 15 multiplier maps that 99,99% of players never used) or Sacred Gold 1 (there are some cool boses , impossible to unlock without multiplier mode , and that sucks)