You should easily be able to achieve Level 6 for the Ironbreaker 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.
He won't rack up kills as quickly as the Marauder because he can't use a bow but as one of your two 'frontmen' it shouldn't be hard to put him in a position to pick up lots of XP.
To train the Ironbreaker 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.