Debra, I'm pretty sure you can drop in and out of games...whether with friends or public. You could start the game over but if you haven't killed him yet, I'm not sure why you would start over.
I don't believe he levels with you...you can always go farm some levels and then try again. Of course, once you beat the game on normal and go on to Nightmare, he will, of course, be much harder.
You can also have someone come in and help with just that fight.
I like to explore and loot so I don't mind roaming around and getting levels before a fight.
It might also be that you need to use a different sset of skills. I will change a skill sometimes before a certain fight, and son does it quite often. The way the skills are set up makes it easy and quick to do that.
Did you pick the skills you want the Templar to use and give him better stuff?