Adventure mode becomes really pretty easy if you cheap out and do one of these things like swimming across the ocean on sneak - after that you'll be very tough and can probably hit average enemies from stealth forever without them noticing, which is incredibly stupid. Or just run around throwing rocks from hiding and killing everything, or wrestle a koala forever, or spend a few days getting hit by a flock of ravens. It's the adventure mode version of danger room.
If you want to do it in a more interesting way, I would highly recommend going as an elf, as the extra speed and peace with wildlife really saves your ass. The first thing you do should be sneaking around a dungeon to find a good weapon like a *iron morning star*, else you're toast. Use sneak a lot, and only attack non-bow enemies in melee combat when you're sure there's no marksmen around to send an arrow through your spine. Only ever fight one foe at a time, no matter how easy ones - I've had a regular hammerman smash my skull in from behind on an adventurer who could kill boss type enemies just holding the direction button. Never, ever be overconfident. For dealing with any dangerous enemy, the order is: don't fight something faster and stronger than you AT ALL, just leave it (a demigod elf with maxed agility never runs into this situation, being practically always faster). Only attack when you have an easy, fairly good shot at a significant body part or better. If you don't, move around. First cripple the stomach and/or one leg to slow them down, then the weapon arm. It takes a lot of concentration and keeping your state of mind serious, but you can have an epic level adventurer even in this version without exploiting game mechanics.
I know you asked about farming, but frankly it's less fun and doesn't even give you bragging rights.