He'll catch up to you quickly enough if you let him get the kills as you make your way through on players 8.
One point of forum etiquette - don't start multiple posts asking questions. If you have several questions, then post them all in one post. I promise that people will see them and respond to each question individually.
Leveling merc to level 25 needs some effort. From that time, they will level with you. Sometimes I let my merc get the kills, only making monsters weeker, sometimes I get the kills and repeat the areas more times (especially if I can kill noticably faster then my merc). Also try to find area, where he gets best experience from monsters.