I will not make a guide on particular challenges, because there are 400 of them!
You need to do challenges on Xbox Live with people that join your game, until you get all challenges checked in a particular section. Sections are linked to the number of players in the game, hence 2-players, 3-players ... up to 8-players. Each section has 50 challenges, and you have to do them all in 2 of these sections (hence 100 challenges in total). Obviously, doing the 2 and 3 player sections may be easier, since they require less people.
1 - It doesn't matter if you do challenges with different players.
2 - The order in which you do them doesn't matter.
3 - You don't have to be host.
4 - You don't have to do the Time Challenges (10 in each section)
5 - Challenges from the Island DLC don't count (they have their specific achievements).