Just to piggyback off Steve here - Tolkas is definitely gold/exp inefficient.
For example, if you’re running three characters around 16-17 and you do the level 17 challenge, you’ll get about 430-580 exp for a 7/8 or 8/8 run, respectively. That can take 12-15 minutes in my experience using full purple-geared characters at 16, 16, and 17.
Contrast that with just running around the level 20 area with your level 16-19 characters, and you can get about 220 exp for a battle with 2 level 20 enemies, and it takes a lot less time.
Clearing the dungeons is also a good way since it’s pretty close to just going to the next highest zone, but my experience is mostly with leveling under-leveled characters. To get your highest guys higher, it’s best to just do the hardest thing you can (e.g. if you’re 20 and want to up, it’s probably best to just heroic the level 20 dungeon rather than do the 17 one on legendary) - that’s how it’s panned out for me so far, anyway.
Another way to get some very effective power spikes is to farm shadow coins via fishing or rapid clears of a two-back dungeon or so - those perk books add up.