O god dude, i did all of those recently with my friend and man it took awhile but im sure it doesnt take 6 hours for each more like 2maybe or 3.( we f'ed around alot)

As for beating faster i couldnt find any way around it so we beat em legitly, all 3 of them. sorry i couldnt help you.

probably the best and quickest way would be to use a modded weapon.

previous poster is right shouldn't take more than 2-3 hours/challenge. Trick here is to have someone host that is much lower level than you. Enemies will spawn at his/her level. If you're a lot higher level you can just mow them down. That's the easiest legit way.(assuming you are l69 or so at present.) If you don't care about legit, just use a stock shield to keep from dying. Even less legit is to use the achievement unlocker on horizon. Your game, your choice.