Go animal hunting. The Animal friend perk makes it much easier, as you can walk right up to any animal (Fire Geckos are NOT animals) and get the perfect shot. Pick a faction, and kill any large group of the enemy. (If you're NCR, kill everyone in Cottonwood Cove [make sure you take Boone with you)
Another thing to do is clear out Vault 3. You will want great armor and at least two companions. My recommendation is to use Arcade Gannon and ED-E or Rex. The reasons- Arcade Gannon has a Ripper and cleans out those fucking tribals like cake. ED-E may not be as useful as Rex, but I used the mod to have all companions at once, so I had every type of fighter. Lily Bowen may come in handy in close quarters or fighting Motor-Runner, Violet, or Cook-Cook because the three have more health than other Fiends. You don't really NEED any companions for taking down Driver Nephi, because if you enlist First Recon to help, Driver Nephi won't get within 15 feet of you if you stay in a line with the team. Use an Anti-Materiel Rifle here, as it takes down most enemies in one shot. It can be GRA or normal. By the way, if you have a companion with a shorter-ranged weapon such as Lily Bowen, Cass, Veronica, or Arcade Gannon, give them a sniper rifle or grenade rifle. Lily Bowen's options are slim, because she can't carry many weapons. Let her go slice with a vertibird sword. This will result in a HUGE amount of XP.