You should easily be able to achieve Level 6 for the Grey Wizard 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 Wizard step in to finish them off and pick up the XP. Using the teleport spell is a good way of skipping through enemies to pick off weaker ones - just be careful to ensure he isn't going to be exposed and targeted by enough enemies that he can die in a single round. Using the extra melee spell on the Wizard himself is another good way to help him finish off Boss enemies.
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.