Since you still low level, I think the best would be focus on equip for leveling.
For leveling solo I think a Minigun [2x Hunter Fly] using Round Trip to leech HP
For damage, if leech not needed, Minigun [2x Archer Skeleton].
About MVPs, I do wanna now what ppl are using to kill them as Rebel as well haha xD
But they probably will use some weapon with Archer Skeleton for damage.
Though, I don't know if ppl are using Dragon Tail (grenade launcher skill), God's Hammer (rifle skill) or Slug Shot (shotgun skill).
Maybe Slug Shot for low tier MVPs works due to them don't hit THAT hard, so you can get a bit closer to use Slug Shot and not get the accuracy penalty.
I haven't tested God's Hammer nor Dragon Tail yet, I hope that someone that have been using this skill help us haha