Derth arena is locked out after they are attacked, which is usually after maj eyal and you are at least level 10; Talk to the relaxing man. After this the mountains, and forest by the lake southwest of derth are marvelous to go through, followed by the ruined dungeon.
The portable chest in the lake through the old forest is excellent for money and carrying things. Get the key before swimming down into the lake.
Daikara should be done after the western beach if you want its timeless content fastest, although the sand takes travel tools to safely go through.
The halfling ruins will help you focus if passed perfectly. The elvish ruins start more difficultly but have a less steep curve. The spellblaze is a perfectly honest name with a trap at the end.
The ultimate challenge on this trip comes after the final ruins.