You should do them within 2 or 3 levels of the above to make them a doable challenge. If you wait too long you walk through them. They are more for the fun of the play than XP, but the drops are good for the level.
I am a GRand Master in Life and to me GH is also little hard to do solo. This is place where you should have friend or two to help you get by it. I have also heard that alot of the bosses there have crazy health. This is a world that I tink is made for all levels but more of a challenge for Grand Masters, but since Grand Masters have stron spells, I would wait till you have better spells if you want to go solo.