Best place(s) to train 50 - 100 for low-tier characters?

From 50-60, do ludi pq.
At 60, go to Tangyoon's Cooking pq (which you can solo). If you need a guide to perfect a dish, I think basilmarket might have one.
Tangyoon's gives decent exp, and, when completed 4 times perfectly (for 8 certificates) you can get a hat and overall with a nice set effect.
Then monster park to 70.
70-75, Sakura Castle.
75-90, Silent Crusade plus romeo & juliet pq
90-94 or so, do the Helisium quests, then pq to 95.
95- 100, ellin forest theme dungeon plus R & J pq.
Hope that helps.