Leves and Behests are the best way to level up outside of instanced raids. Casual grinding should be done only, and I mean ONLY if you have no other options available. Just always be near a camp (or have fast access to one) so you can join Behests. There's absolutely, positively zero reason why anyone should ignore Behests. They happen every 30 minutes and is practically a free Guildleve.

Still, most good exp spots can be found near camps. But if simple grinding is getting boring, I suggest getting a Behest party that just sticks together to do behests as they show up every 30 minutes and take roughly 5-10 minutes to finish. Once you're done, break out a DoL or DoH class and level your crafting until the next behest starts.

There's always something you can do to raise levels. But once you hit level 20, it's suggested that you start getting into Guildleve parties. Get some friends and form a Static since this game's party search/party flag options aren't properly made and utilized. Even a small party of 4 with only 4 available leves per person and alternating with Behests is a great EXP session, especially during the 20-30 slow spot where there's very little in the way of same-level enemies.

So, long story short, only grind if there's absolutely no options left available. And if you have to do this, make sure you're able to get an EXP chain going for bonus EXP or else it'll feel even slower. Use those links given to find where enemies of your current level are hiding, but always be sure to do those behests. You'll be kicking yourself later if you don't. They happen at every camp everywhere in the world every 30 real-world minutes. The battlewarden appears on the 00 and 30 minute marks and behests begin 3 minutes after. And if the game says you need to wait longer for the next behest, just spam the warden until you're able to join because your personal behest timer is also set to 30 minutes.