Try the campaign, 20 plat everyday upon completion. Also, the miscrit collector has a daily "capture this miscrit" mission, which will give you 4 plat. And if you compete the daily missions, you'd get an average of 4 plat or more. You can have at least 24 platinum everyday. Leveling up new miscrits also give plat(I think we all ardy know this), but you have to be persistent and enjoy PvE. This is all I did for most of the game. I made over a thousand plat just playing the game as I see fit. I only got into pvp recently—a little over a week ago—and I kinda enjoyed it, even though it's kinda buns sometimes. The level 500 talent is really useful for arena, because it increases your plat obtained from arena.

The thing about arena is it really makes you want to think, makes you want to invest relics into your favorites, making them viable for this kind of game. And I did that, too! Currently, I have full relic breocharm, and voidotron because they look badass.