I can't seem to figure it out.

thresher/giant thresher for speed. where CB is more of the mercs dmg.

cryptic axe for damage, where damage is more of the mercs damage. col volgue is ok, but min dmg is low. and str req is a bit silly.

higher attack speed is generally better for a number of reasons

thresher or giant thresher for ias
more ias more ll