For those that dont find the answer, you can't make the merc the main assist or puller, those have to be human players. (At least today they do.) If you molo setting yourself as puller means the merc wont follow you he stays in a spot and you bring the mob in. The merc will aggro when you are close. You may have to take yourself off puller to reposition the merc every once in a while.
For Truth, Justice, and the Norrathian Way!
Tinkering is a trade skill which will not prevent the mobs from hitting you.