Here's a
link to some information regarding EXP in D2. I'm pretty sure it still applies to HU, except for the "reduced expereince after level 70"
1) If the monster level is more than 10 different from yours, then you gain 5% exp, if it's within 5 levels then you gain 100% exp. But, if you're above level 25 and the monster level is higher than yours, then you gain exp equal to your level divided by monster level. So if you're level 25, and the monster is level 100, then you gain 25% exp, if the monster was level 75, you'd gain 33% exp.
2) When any party member (even you) kills a monster, you gain exp based on your level compared to total party levels. If you're level 5 partied with a level 100, then you gain 5/(100+5)% exp, or about 5% exp. If you were level 25, then you'd gain 25/(100+25), or 20% exp. If you were level 50 with another level 50, then you'd gain 50/(50+50), or 50% exp.
3) The player giving the killing blow gets a 10% exp bonus. This bonus is based on the inflated exp of the monster with many players in game (so it ignores rule 2), but only applies if there are players in your area (2 ppl in Blood Moor)
Rule 1 and 2 are cumulative, so your level 5 would get getting about 5% of 5% (0.25%) experience per monster killed by the level 100 char. I suppose if you were in Hell Tundra, then you could level a bit. By the time you were level 50, you'd be getting 5% of 33%, (1.5%) of the experience from kills.
I guess you just need a dense area for monsters, so maybe Tundra (if you can survive the catapults), River of Flame (there are some rare debris flames that can hit you), or maybe Flayer Jungle.
Some people get their kicks from rushing a char to Hell as a level 1, I hope you had fun.
