Note: You must have completed all of Professor MacDougall’s missions for this one to appear.
Go to the police station and walk into the yellow cross once again to start the mission. Once you're in the back of the van with the Gatling gun, keep your itchy trigger finger at bay until you Dutch's men start their assault.
Keep that gun firing at all times and wipe the floor with any enemies (remembering to go for the horses themselves as they're a much easier target than the men). When you start driving off again, keep an eye out for any red dots left on your radar and finish them off with a hail of bullets.
Watch out for the next part as you'll encounter a few dynamite throwing enemies further up ahead. You can't afford to take too many hits, so keep the fire button pressed and take out the horses to make the enemies drop like stones.
Keep your eyes peeled for the truck packed with explosives that heads straight for the cavalry, you need to blow it to bits before it reaches them! Keep your eyes peeled for more red dots on your radar and whack them all until the next cut-scene triggers, leading you immediately into the next mission.