The Kingpin achievement in Red Dead Redemption worth 55 pointsFight off 8 unique players during your time on top during Land Grab in Multiplayer Free Roam. IIRC Those are all the achievements that explicitly need more than 2 people to acquire. Although you will probably want a full group for the Advanced Co-op missions.
The online achievements aren't terrible difficult, just need to boost all the 3+ ones with a coordinated group (especially the poker and Liar's dice ones) and grind a lot to get to level 50.