First of all: Thank you Chapuz. This tutorial has made me rich
I've been using this guide for some months (with approx one hour playtime per day on average) and right now I don't see how I could ever become poor again electrum-wise.
One should be prepared for a lot of grinding. The Limitless Speed stage was annoying at times (especially since Firecell seemed more fun to me). I moved to The Bone Collector relatively early, which was ok once I upgraded the Otyugh. That seemed to make a big difference. Alternating between arena and FG grinding with these two decks offered enough fun to keep me going. While playing the Bone Collector, I step-by-step became aware of which cards in it need upgrading before others (look at Guardian Angel early for more attack power!), which in turn provided motivation for beating up some FGs with Limitless Speed again.
During this process, I naturally got rare spins, in which I mostly aimed for Shards of Bravery, as those were the only ones needed for Voodoo Bravery. Also, I want to use them again for The Brave Malignant Ball (still missing some other shards though...). Going for SoBr was a good idea I think. Once I got 5 of them and upgraded 6 Basilisk Bloods, money came rolling in. No stopping it. The rest of Voodoo Bravery and The Bone Collector was upgraded ridiculously fast.
With Voodoo Bravery finished (I use the Nova version, seems more consistent to me), the only thing I lack in this game now are rares.... time to destroy some arena again