Make sure you have cleaned out the whole of Darion already (with a few exceptions). But if you are level 10 then you might have done that already. Basically, there is a big step up in difficulty in Freedom Isles. The hardest part is actually the main continent in the Western Islands, which is pretty much the first thing you see. And remember that enemies listed as 'Slightly Stronger' or 'Strong' are beatable with your superior intellect and ragebox.
In terms of units to use etc, please have a read through the Battle sub-board. Plenty of useful advice there.
Oh and I've just finished my first game, which was as a Hard Paladin, so it can be done!