You should easily be able to achieve Level 6 for the Marauder simply by doing quests in the first area of the game. The last settlement quest for this area is Recommended Level 7 so you'll need to be at that level to stand a chance anyway.
If anything, you'll need to share out the kills to ensure your other classes progress as the Marauder's ability to use a bow and melee weapons means he can mop up kills a lot faster than the other classes you start with in the core game.
To train the Marauder and get to the next level you'll need gold - off the top of my head it's roughly:
Level 1-2 - 100 gold
2-3 - 500 gold
3-4 - 1,000 gold
4-5 - 2,000 gold
5-6 - 5,000 gold
Simply keep grinding through quests in the first region of the game and you will have no problem progressing this guy.