How can I farm experience?

Start a LAN game, check the 'Reroll world' box. All the enemies will be respawmed (possibly allowing you to spawn side quests that didn't appear in the original world - I haven't cared to check for that yet). You'll lose your character's original world. but you'll have tons more mobs to kill. It's kinda boring, tbqh - that level jump isn't as hard as you might think, and the only real annoyance I've had is that items don't drop ID'd anymore because they're higher level than I am. Playing on Normal - my Veteran character got devoured and spat back out trying to do the same thing.

Doing dungeons under requred level is best way to gain huge chunks of experience.

Thats also main reason why to play your future heroes at highest dificulty :)

Also you can recreate ACTs and replenish them full with enemies once you reach ACT II and futher. You just need to travel from one city to another to refresh them and many repeatable side quests. [This will grant XP only 1st time though, granting FAME instead if repeated]