I have read before that only those who die on the battlefield would enter Valhalla, but this seems almost unfair to soldiers and warriors who were good enough to never be defeated. Is this just a myth to make the Vikings seem more fearless? As surely an unblemished combat record would allow you into the hall of warriors?
The short answer here is "No". Valhalla was reserved for those who died in battle, those who died of sickness or old age went to Hel regardless of their bravery in life.
However, how Hel was perceived by the Vikings/Norse men is a matter of debate. In Baldr's Dreams it is described as "Hel (the giantess who rules over Hel the domain) had decorated a lavish feasting table when she waited for Baldr to enter her halls". The idea that it was a horrible place similar to the Christian Hell may be something that was influenced by the spread of Christianity.
You might also be interested in this post from a few weeks ago where a similar question was asked and answered.
"The greatest of all is this: that he made man, and gave him the spirit, which shall live and never perish, though the flesh-frame rot to mould, or burn to ashes; and all men shall live, such as are just in action, and be with himself in the place called Gimlé. But evil men go to Hel and thence down to the Misty Hel; and that is down in the ninth world."
A part of chapter 3 of "Gylfaginning" by Snorri Sturluson (http://www.sacred-texts.com/neu/pre/pre04.htm). It tells of Gylfi, a swedish king who travels to Valhalla to learn of the way of the gods.
But like has already been said in this pots it was written by a christian man (Snorri Sturluson) some 300 years after "the fall" of Ásatrú.
How would you define a battle? Would a courageous warrior who survived long enough to die of wasting sickness (I'm assuming cancer existed back then), while still courageously going about his duties until the end be considered to have died in battle against the disease, for example?