Blessed aim you don't actually use - if you bind vigour then you can swap between that and concentration when you're running, the speed increase is pure <3 Holy shield is great but its not as important as its a synergy - 1 point in it then +skills will sort you, and you can put more points into it after synergies are maxed if you want.
In case mana is a problem, I would even max one of the synergies before maxing BH itself. That adds damage without increasing mana costs. However, Insight will help a lot to minimize mana issues and runewise your merc can (and should) wear that runeword from L27 on, e.g. in a bill (ethereal if possible).