There are currently 8 monsters that utilize the Normal Element
As of the 7/16/2012 patch, all Normal monsters share the following qualities:
When battling a Normal-based monster in the wild, your most effective fighter will be of the Plant, Water or Fire element, as Normal is weak against all three.
This battle combination will force your attacks to have a 1.5x multiplier attached to them. Since no element is strong against normal, this is technically the hardest element to fight, since it has no direct opposite.