Well, I figured most CoAs are already Ber'd since it's standard practice. And DR% is good on the merc too. However if you want to spare your runes, ias/ed jewels would be nice too (not 15/40s obviously). Check the ias bp for your merc weapon from the German calculator (just type "diablo 2 ias calculator" in Google, it should be the first site), and insert jewels accordingly.
My might merc has a Toll/CoA setup with 2 sol's but then he has a gladiators bane so he total PDR of 34 and a MDR of 20 some regular DR.
With a Toll you might consider a 15 ias / xxx jewel as he will suffer very much under decreep without it.
It does kinda bug me that he tanks hydras much better then I do tho
But without the gladiators i guess a ber and or jewel setup would be better.