[Edi]

Stay away from the glamour nations (Vanheim, Helheim, Tir na n'Og), they tend to be really successful for the AI, since they have better than average troops. Niefel has the problem of getting its troops swarmed against the AI, but if you take a Nature bless (e.g. N9E4 or N9E6), you should be able to reach a critical mass of Niefel Giants and after that it doesn't really matter much who you face, the cold aura does them in.

Middle Era would perhaps be better to start out with, since you have a greater variety of better baseline troops with many nations. Using the Better Independents mod (see sig) could also help you some.