Having just soloed my way through 19 two days ago I can confirm that the burnt out fields are excellent. My 19 crusader breezed through the earth burrowers, they were blue and white cons... at 20 green and blue. You will get the occassional bear or wolf near the fields, bears are typically white cons though I did see a red or orange once or twice. Stay on the fields themselves and no worries.
On the hill next to Gnollslayer keep there are a ton of bears and wolves. Most are blue, some white and I was unlucky enough to have a lot of yellows for a few hour stretch. Those yellows are a real son-of-a-beotch to non casters and those that can't kite... not that there is a ton of room to run either since there are so many. My tactic as a crusader was get close enough to use a DD spell (righteous anger), slam it, run close to the moat so I had it behind me preventing a mob from coming up behind me... and wear the bear or wolf down.
I also found fighting in 3rd person mode to be a big help in case an aggro mob did manage to get close enough. You could be at 50% and have that even con down to 25% and thinking ok... got this one in the bag, drop it, be at 25% and still taking damage only to find out there is a yellow con bear behind you... Third person mode lets you know if there is one out of your peripheral vision and if its time to boogie.
Be careful around gnollslayer, I've found that the sentry there does scant little to save your **** if you are in trouble!