not gunna lie to you man but diablo is a 99% grinding game buy the biggest grinding game company in the world, blizzard, although to try and fiz your dh have you got the passive skill that increases your crit chance by 3 per second its really helps seeing as DH's are very weak characters so they focus on crit chance and damage along with attack speed you should focus on these if you want to progress into act 2,3,4 you will deffinetly need a team to take on anything above act 2 mainly comprised of mage monk and barbs (no WD's they suck imo no offence) you will struggle on act 2, also even though you and your bros chars are same level they are much different soley because of the class, if i were you try again with a different class untill you can find a inf DH i couldnt do it first time but as soon as i made a monk i could farm act 2-3 to help fund a DH thats could tackle inf DH's are bad first time chars imo
sorry for long post i tend to ramble but i think its worth a read