Not sure if mercs get + skills, I have to look into that, but, if they do not, then the Spirit runeword is useless on a merc.
Try making a aura merc:
Dragon - armor/shield
Dream - helm/shield
Lawbringer - weapon/ 6x 5/5 facet sword (facets to match merc elemental)
Phoenix - weapon/shield
Any of these in combo with the rest, can cause some serious stacking of auras and damage, but, once again, depending on your character and main purpose it might be better to go with a act 2 merc.