The level disparity in parties is an issue.
On thing I have noticed is that if you go to a safe house that is within you level range (which is dependent on the location) and party up, you will be partied up with people that are within the same level range as you.
The level is displayed in the top right when you look at the map.
I don't know if that is guaranteed but it seem pretty reliable.
I do agree that the game gets pretty routine however. Even the encounters and side missions are pretty much the same.
The game is best played and most fun when you do get 4 people that are the appropriate level doing a mission that is also the appropriate level. If you are 1-2 levels higher than the mission however, it gets pretty easy with 4 well organized people.