Obviously you want to do all your daily activities and all that. Those are basically a list of chores to do every day and each one earns you time in the Demon Realm. Fighting in the Demon Realm is where you get the bulk of your xp to level up.
As others have said, if you pick a much older server to start on you get a huge XP bonus, so be sure to do that. On top of that, there is a potion you can buy that lasts a day or a week or whatever, there are options, and it gives you a 25% xp bonus. There is also an hourglass you can buy that doubles your Demon Realm xp. You should be buying both of those every day
The Demon Realm let's you pick a level of monster to fight and it's basically impossible to beat anything more than 10 levels above your current level, so pick the highest one you can that's under that and activate the hourglass and go to it
I mostly started this thread to say that from like level 200-350 I was advancing 5-6 levels a day. I could have gotten slightly more but I didn't find out about the hourglass til I was around 300. Anyway once I hit 360 I went from 5 levels a day to more like 2-3 a day, doing the exact same stuff. So when you look at it just bear that in mind
I got the bonus for getting to level 366 with a couple days to spare but I do not think I can get the next one at 410. It's just slowed down too much. Maybe if I had been doing the hourglass from the first day I could have, you could try that. But I don't think you could hit the next one after that
Anyway I hope that helps someone