I've only spent about 1000 NG on that game because of limited special sale/deals but other than that I've slowly but surely accumulated Magatama from just doing the daily quests. Saved most of it up for whenever there was a good Gacha deal for free magatama and used it on that. I remember there was a moment where there were a bunch of them at the same time and I blew about 160 free magatama and got a heck load of 5 star Senki from them.
Being part of a decent guild helps to get those medals or whatever they're called to get those other 4/5* Senki and whatnot.
And the 4/5* Senki/Inhumans you get from drops in the daily rotating 'monster' schedules aren't even half that bad except that you can't use them in guild wars.
There were a decent number of events which just required you having completed the campaign(s) and you got 5 star Senkis for that as well.
However, I haven't played it much lately so I really don't know it's current situation or if it's even doing well because I never see it in the top ranking anymore.
All in all, not a bad game I'd say.