Besides some elf (i don't remember who) injured him and if he can be injured,
then his body can be destroyed. So the elves just lacked the military might
and/or luck (and perhaps technology ;) to destroy his armies and fortresses. At
least I got the impression that they were very close to victory in that battle
and that it might have resulted as winning the war.
I was also thinking what would have happened if they had won their war and
regained silmarils.. would elves have continued to be the dominant race in
middle-earth?