Okay, you are beginner in Skyrim world, and want to level fast? Kill some Mammoths? Maybe some giants? Well, it is possible. This is not really an easy glitch. You need to have some magic, bows and arrows in order to complete this.
Also, you don't need to be high level player to complete this, to be honest, you can be level 2 and you will still be able to do this glitch.

The thing is, mammoths' AI is stupid. They are too slow for you, because they are big. You can go around one rock, or anything like that, and cast spells at them, or shotting them with bows and arrows. Of course, they will try to get you and kill you, but as I said, they are too slow. They will die, and you will beat them.

Another great method of killing big monsters in Skyrim (Read: Giants and Mammoths) is to get them to some doors they can't get through. You can do this with any big and dangerous things in Skyrim doesn't matter. Only that matters here is that they can't get past that doors or passages. They will trying to attack you, and will fight air. You just stay near them, so they can't reach you and can cast some spells on them, or shoot them with bows and arrows. This method is easiest than first, but it can be hard luring monsters if they are fast.

I recommend trying this with Mammoths or Giants.

And last thing to add: You can always try good old "Hit and Run" method. Just hit them, then run, repeat this until you kill them. But beware this is hardest of these three methods. Easiest is one with doors, so good luck hunting these mobs.