The wood elf "gaenor" is something like a joke by bethesda, because many fans complained how annoying the wood elf Fargoth was. So they decided to put in a real annoying wood elf. <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />
He is not invincible, but has a very high luck level. (So he can´t be hit on normal way). In order to kill him you need also a very high luck level. Boost it up to 1000 or so. You can do this by making quite alot of luck potions and drinking them simultaniously. Also some strenght potions could prove helpful.
Gaenor can´t be easily killed by spells as he can reflect most (all) of your spells.