Instant kill spell
Turn the difficulty all the way down and go to the Skingrad Mages Guild. Seek out Vigge the Cautious and purchase the Weakness to Magicka spell from him. Then, seek out Druja the Argonian (also in the Skingrad Mages Guild) and buy the Minor Wound spell from her. Both are novice level spells. Go to a spellmaking altar, either at the Arcane University or Frostcraig Spire, and create a spell with the following effects in the corresponding order:
Weakness to Magicka 100% in 1ft for 1 sec on target
Drain Health 100pts in 1ft for 1 sec on target
The end spell will be an apprentice level spell and requires less than fifty points to cast for a level one mage Breton. It is vital that you do these in the correct order, as the effects will be applied in the order you add them. If done correctly, this spell will first weaken an enemy to Magicka 100%, so that you drain 200 points of health from your intended target. It will last only one second. However, if you turn the difficulty all the way down, no enemy will have more than 200 points of health (not even Xivalias). Once a character's health reaches zero for any length of time, they will die. You can take down anything in one shot with this spell, even Mankar Camoran, who is 20 levels above you.