To get an act2 merc up to 200 Str without any additional gear, you need to be over level 90 (I socketed Bonehew with two rare jewels that had +ED before I realized this).

Don't go around monster killing to level up your mec if he's a couple levels lower than you, just strip him of his gear and buy a new merc with the highest level (should be your clvl - 1)