All monsters have a defense rating that is used to counter both the physical and magical attacks. For instance, a "Sentinel" is substantially armored compared to other monsters in its level(Maplestory powerleveling) range. Against weapon attacks, it has a better defense, then makes it hard for most characters near its level(MapleStory Mesos) to launch a sufficient attack.
Maplestory Magicians, on the other hand, are less adept at killing monsters which employ magic defense including "Luster Pixies", who enjoy lesser vulnerability against magical attacks. Other monsters have elemental strengths and weaknesses that make them, more or less, defenseless against certain elemental attacks. A case in point is the "Fire Boar", who is weak against ice-elemental attacks, yet strong against fire-elemental attacks.