20 should be easy.
Use Kazordoon Mines. Learn every mine out there, explore every sqm of them. It's probably my favorite facc spawn cus u can be there for as long as u want - with many chars at a time.
There are so many mines and they are for all kinds of lvls. The deeper you go, the higher lvl u need.
I've botted 12 chars alone in all kaz mines, from lvl 8-100+
On bottom floors it's some sick exp at the guards.
Other spawns (in no lvl order):
- forest fury (carlin)
- ghostlands / isle of kings (carlin)
- poh necro house (it's actually good exp ; watch out for GS when refilling)
- ab trolls/thais trolls
- swamp trolls venore
- any rotworm cave
- folda (ice islands)
- undeads under mount sternum
- orc fort
- shadowthorn
- elvenbane
- hellgate
- kaz/venore coryms
- venore/carlin dragon lair
- crusader helmet quest dwarf guards
- cyclops south thais, cyclops mount sternum, cyclops poh
- venore swamps (some new place too with gloom wolves and such)
- trolls under dwarf bridge
- dark cathedral
- mintwallin
- orcs in ancient temple
- combine Wyvern (near orc fort) and the small Kazordoon dragon lair, or Wyvern + Rorc
- amazon camp
- drillworms kazordoon
if paladin:
- banshees in banshee quest
- demona
- poh GS
- lost husher caves
Just to name a few.
Once you are lvl 30-40+ on the chars there's more variety of spawns u can go.