If you just got good plvler (by good plvler I mean a wiz that can one hit Party Penon, altough I think nuker can do it too)
- Levels 10 - 27 at Ongs (Many good spots at there but might be crowded)
- Levels 29 - 34 at Sungsungs (River is a good safespot)
- Levels 35 - 42 at Mujigis (Any spot at sides and at top of Karakorums enterance will do)
- Levels 43 - 50 at Penons (Good spawn rate, passive, no statues = Perfect)
Also, if you get a bard for Ongs and Sungs, that would be great, but at penons, same level wiz as the bard was will do much better.
And also notice that if you are going to get plvld at Ujigis, rember that they won't have Giant Party monsters. And Penons can be pretty much full of goldbots.
With 0 gap you can pretty much do this in a week if you just can leave your computer on overnight without dc (Premium needed), but it really depends on your plvler, as everyone knows, 110 lvl wiz does much better than 76 one for example.
Hope I helped...