monster should be divided into groups, each groups will receive increased damage from certain elemental type and reduced from other type.
creature,undead,fire,ice
creature:wolf/bear/baku/minotaur/dark bear/... 30% more from fire, 15% more from poison 15% less from lighting 30% less from holy
undead:zombies/specter/mushrom/skeleton/... 30% more from holy 15% more from fire 15% less lighting 30% less from poison
fire:Rhinoceros/hell hound/ettin/ciclop... 30% more from cold 15% more from lighting 15% less from holy 30% less from fire
ice:meti/cristal golem/... 30% more from lighting 15% more from fire 15% less from poison 30% less from cold
this could be aplly to skill to:
skill from the lighting and earth skill tree are more effective against ice
skill from the fire and posion skill tree are more effective against creatures
skill from the ice and wind skill tree are more effective against fire
numbers could be change, the idea is to improve the dinamic of the skill making them all good in some way and making the game less monotonous.