Each monster has a designated weakness. Always try to exploit this weakness for maximum DPS.
Lesser Demons, for example, attack using Magic. This makes them weak to range (in general- bolts having the highest accuracy against them.)
Magic and Melee will both be okay, but won't be as effective as range (note that at higher levels, you have a bit more flexibility)
In comparison, Hill Giants attack using melee- this makes them weak to Magic attacks (Check out the Combat Triangle) and thus magic is more effective against them than it is against Lesser Demons.