diablo-3
If I've skipped to Act II as soon as I can, but I come back eventually and play with a level 3 player, the mobs are very easy.
Are they providing me with less XP per mob then I would've achieved if I were level 3? Are they providing my friend with less XP then if he were playing with another level 3 player?
Many people are discovering this problem and very clearly the easiest method is going to be to try to work with people who are on the same quest as you.
Given that will not always be possible the choice comes down to helping your friend catch up or speeding your friend to your quests and allowing them to replay them in their own time.
By maintaining a large friend list by joining a clan, playgroup or adding your name to the spreadsheet of a forum you participate in, you should be able to find someone who is in the same general vicinity as you. Unfortunately there is no method at present to group friends or to edit their names into something more familiar to you (eg a forum ID that you recognize instead of their BattleTag/RealID) so you should maintain a list somewhere to help you recall.
A great number of people (myself included) are deciding to solo Normal mode so that they can take all the time in the world to experience all the details and nuances before worrying about co-op play.
With more experience I've also taken the following action; play one character as my "get through the game, unlock things and get good loot" character and started another that I quickly levelled up. In my case my primary is a demon hunter and my secondary a barbarian. I get to play different content but still get to keep pace with my friend.
I also installed skype to give us voice chat, which helped a lot.
You will not lose the gear that you have gathered.
You will lose save progress if you start at part 1, you can go back to the quests you already done (In your example 7 to 9).
The way the party system works is that you always enter the game at the level that the host is playing at. If they are further ahead of your character personally you will join them at part 7 (in your example).
When you go back to your own personal game, you can start back at part 1 but it will prompt you to save over your spot in the game. You can continue on from part 9, but you lose out on the story line in between.
What you can do is have your friend join you in your game and help you kill the butcher and jump ahead after that is done.
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Best Answer
Presumably, the reason they're easy is because you've outleveled them. Monsters below your level provide less experience, as seen in the table below.
Level difference is calculated as (mob level) - (character level).
Bonus XP/kill from gear is only applied if the monster is at most 5 levels below you.
Source: Click to Loot