I had the same problem. I finished Nightmare at 47 and had to grind the last few levels to start Hell.
Here's what I found that was the best combination of good exp, non-tedious, and actually pretty fun.
- Choose the very first quest in Act 3.
- Run to the first part of the "Light 5 signal fires" quest where the guard calls to watch out for the meteor and a pack of goblins runs down the stairs.
- Go for the highest "Massacre" bonus you can get. Once you stop, leave game and try again.
I usually got massacres of 70-90 almost immediately followed by a second one of roughly the same size (never was able to chain them together, was so close though a few times). Each run gave me about 2 bars of exp.
It works because in the first 3 areas you get a 90%-ish chance of a large goblin pack spawning. These are very easy to dispatch quickly and move forward. You'll meet the first one right at the top of the stairs. Then you'll either be able to go up the stairs on the left to a platform, or around the battlement to the right. Go right, there is almost always another large pack of goblins. Fight through them and you come to the second large area of the level where there is a third goblin clan.
This method got me from 47 to 50 in 30-45 minutes.