Well, my merc has actually rly bad equipment - non eth reapers, duriels and crown of thieves.The final wave pretty consistently rinse my merc on higher player settings- pretty sure it's because p5 is the threshold for my hits putting them into hit recovery.
What gear are you running on your merc?
He doesnt have much troubles with p8 lister wave, unless they are extra strong. Or cursed. Or might/fana... or any combination of them Then it takes quite a lot of teleporting and splitting the group.
I think that helmet is ok spot for socket, I actually made one socket to guillame, keeping the rest for final respec to conc barb. I found decent all res - pdr circlet, hopefully ill be able to make 2 sockets in it...