Elemental def down is one of the most powerful Status ailments, so almur first. Every green stack will get huge benefit from it.
If you have the ascension materials, the rat next, if you are lucky to pull costume for kadilen, you should really consider her, but maybe others have better advice in this.
Then in my honest opinion it’s got to be Kadilen in costume if you have the mats to max her.
If not start with Almur
Whichever hero you choose good luck
As always, it depends on what your priorities are. That said, once I pulled and leveled Almur, my titan scores shot up. I then added Ratatoskr and they went even higher. I run Tarlak, Almur, Buddy, Liana, and Ratatoskr and let the gems do the work. I use that stack on both titans and raids with a lot of success. So from your 4*, Almur would be first followed by Buddy then probably Brynhild. 5* I would personally go Rat over Kadilen. Good luck whatever you decide.
Thank you all for your insight.
I will start with Almur first, and then maybe cosider CKadilen next, but that somewhat depends on the amount of sturdy shields in stock at that time.