I usually ran through the Illithia Delve level 50 six times a day when I was leveling classes. I could do every room in just a few clicks, get 2 XP per battle and collect some loot at the end. It’s pretty fast and nets 104 class XP per day. Even at higher levels, that’s 1 level per day at a cost of 50 gems.
Using the Thursday class events does give you more levels in a shorter time, but it’s also more expensive and you have to wait for the right class to work its way into the rotation, which could take a while.
My team (if you’re interested): Hero (any class and non-red weapon), Tesla, Tai-Pan, First Mate Axelubber, Spirit Fox banner.
Edit: Add to that doing dungeons / adventure board / underspire. That’s another 38+ class XP per day without a cost.