First get high enough conjucation level so you can summon a really strong monster. I recommend a lich, or storm atronach. Then get high enough illusion level so you can use ghostwalk, or another spell that can make you invisible for 30 seconds or more.

Now the fighting part. First hit the opponent you want to fight, then summon the monster you chose, (see above for recommendations) and then turn invisible with an invisibility spell that you chose.