The achievement "So Much Blood" requires you to gunzerk for a full 90 seconds. I want to know what skills I need to put points into to accomplish this. I would also like to know what is the best way to get the achievement.
As taken from the youtube video, you obviously want to focus on the Gunzerking Skill Tree.
Skills that increase time:
Skills that increase clip size (to shoot faster and longer without reloading):
It is also advisable to bring a class mod that benefits both of the time based skills (Last Longer and Yipee Kai Yay)
As for killing a lot of enemies, I'd advise you to visit Fink's Slaughterhouse. It's a great small arena that spawns waves of enemies at you, perfect to help add to Gunzerking. You want to bring large clip and high damage weapons, such as Assault Rifles, Shotguns, and SMGs. Rocket Launchers are also advisable, as high damage can easily clear out waves of enemies, granting more time for Gunzerking.