alot of these people on here have the right idea in regards to leveling up. but i have yet to see on of the simplest ideas on here. so here's the easiest way i got up to level 52 archery in about 2 hours. (this is a somewhat boring segment so if you hate taking time to do things then this isn't for you)
1: get all the arrows you can.
2: head to whiterun.
3: go due east or west whichever and find the mammoths. (they will be accompanied by giants kill them first)
4: fire an arrow at a mammoth.
5: run to the nearest rock ledges and get to the highest point the mammoth will follow you and stand under you most of the time.
6: fire until the animal dies.
7: fast travel to nearest city buy arrows.
most of the mammoths are accompanied by 1 or two giants and the mammoths travel in pacts of 2-5.