Instant kill spell
Turn the difficulty all the way down. Go to the Skingrad Mages Guild. Seek out Vigge the Cautious and buy 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. Create a spell with the following effects in the following 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 important that you do these in the correct order, because the effects will be applied in the order you add them. 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, but if you turn the difficulty all the way down, then 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.