Crafting G1 mods might be most efficient way, but damn it's boring. I just did The Dweller from Level 1 to 5 using only grade 1 mods. Took around 300 if I recall (200 or so to go from level 4 to 5). It was like watching digital paint dry, only much less exciting.
I had The Blaster up to level 4 but he's back down at level 1 now, and there's no way I'm crafting another 300 mods to back up to level 5, even if I do have enough Nickel. Bounty hunting might be slower, but it's certainly a lot more fun! Plus x5 MC + special effects will mean crafting 5x300 G1 mods... screw that!