If you expect DPS from your merc, I think you can consider Last Wish + Grief.
I only have two accounts, so Witchwild String works well for me. I think it's better than Buriza-Do or Demon Machine for killing bosses and heralds.
We don’t realy need more dps on the build but since we already have conc aura on ourself with mist build the merc would get a decent boost.
I would mainly run it for the lifetap and crushing blow. The full magic build doesn’t run leech (or has to drop some magic dmg/fcr for it). It also doesn’t have crushing blow (it sucks anyway on range atacks).
A5 merc with 2x last wish adds both of this things.
Since we want to run 2 defilers and don’t use amp demon or other curses, lifetap is a great addition and will be up a lot, adding huge sustain to ourself and the merc himself.
The crushing blow helps with bosses and t5 heralds and is a great addition since we don’t run any cb on the build.
The level 34 might aura is just icing on the cake. Even with our low phys dmg that’s a lot of ed. Together with our conc aura and defilers that’s some nice dmg buffs four ourself + merc , letting him deal decent dmg himself .
I think frenzy last wish merc is a great addition for mist lock. Adding the missing CB to to build, big, Ed% and lifetap for our free „curse-slot“.