magic isn't much use once you move on from rats and rogues...
Actually, I've felled many an ulbernaut with just the basic spells (the only ones I have). Of course, my strongest one only does about 0.7% of damage to the beast, so it takes a good while. I have to bring 20+ mana potions.
While that may seem pitiful, I would have absolutely no hope of taking one in physical combat. I doubt I'd even get a chance to land a hit before finding myself looking at the Death Guardian.
Considering the time it takes to go get the potions, get all the way out to the "den", slay an ulbernaut, and return, it may be faster to fight fanatics or mercinaries. But I wouldn't get the satisfaction of beating what is supposedly the strongest beast in the game with a relatively weak and new character. Also, if I bring 50 potions, I can take down two per trip, which increases the efficiency a good bit.
As for training by doing a job, as I understand it that's even slower than fighting. Also, I don't believe alchemy is implemented yet. I wound up starting with rank 8 in it too

Oh well.