You'll need 10,000 gold to train a warrior from sixth to seventh level. This will likely be the Marauder as he can rack up kills the quickest. Simply throw them in to battle and if you really want to focus on a single warrior then weaken the boss monsters and then let your chosen warrior finish them off so they get the XP. It can take a little while to grind up to 7th level but it shouldn't be too hard.
Once you hit sixth level everything goes up a gear so make sure you are stocked with provisions and healing scrolls and potions. On the flip side, it also means there's a lot more experience points to go round so while the chances of dying are a bit higher, the progress through the levels can be even faster if you face a lot of enemies in a single dungeon.