Convince one of the NPC's to be your follower, this is important. Find an early equip.

Go up as many levels as possible. The higher, the better. Once you are here you will continuously fight enemies, do not worry about their HP. There is a chance for instant kill or charm on the enemy that you want to attempt to invoke. After a while, you will either accomplish this or die. If you die you will stay at the same level, have your follower heal you and then continue trying to kill enemies. This will give you tons of EXP and good Items.

After all of this we will now fight a legendary boss and try to get 3 instant kill checks before you die. Just spam attempt this and eventually you will kill it. You will get an amazing item as well as EXP and Gold. Before fighting this boss if you have any equipment in your inventory, you will want to craft or sell in case you die.

You are now powerful, fast.