You should easily be able to achieve Level 6 for the Elf Waywatcher 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.
Ironically the main attacking spells don't do much damage and you'll pick up kills more quickly if you get him a decent dagger or sword. Then you need to weaken boss enemies with the Ironbreaker and Marauder and let the Waywatcher step in to finish them off and pick up the XP. The bow isn't powerful enough to kill anyone except basic enemies in a single shot, and the melee is even worse, so it's definitely a case of picking off basic enemies and letting the stronger warriors wear down the boss enemies. Then let the Waywatcher finish them off for the XP.
To train a warrior 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.