Wolfsreiter

I honestly can not fathom the thought process behind t1 and t2 and the buildings at all.
You are investing so many card slots in stages that are a fraction of what matters in rPvE.
You really want to cut  down on especially t2 and t3 units and definetly only take furnace of flesh and decomposer. Decomposer only if you can not afford Cultist Master or Shrine of War.
Nightguard and Witchclaws have strengths that are mostly irrelevant to you in rPvE. Nightguard is an L-Counter and is mainly strong because of her ability. The ability is not worth on anything you face in t2 in rPvE 9's. Twilight Hags and Treefiends you can maybe make an argument for at t3 but thats at best a semi viability against 2 specific units. Witchclaws have their occasional use case in rPvE, but only in level 10's when the spawn building at t2 is very hard to reach, sso again nothing that you'd want to pick for your deck, especially not for one that is made for various use cases.
If you want to start shadow t1 I highly recommend to either play Nox or Forsaken with Motivate and a presetup Soul Splicer. Alternatively Phase Tower with Dreadcharger or Skeleton Warriors are decent, you can probably play Witchclaws with that as well, though its not optimal.
Let me be blunt your t2 is overloaded with units, but lets start with the good things: Lavafield is a good pickup as a card, decent AOE damage especially for a t2 spell. Windhunter is a theoretical option with Ravage but not my preferred one because. What I'd recommend doing is either go shadow phoenix with Embalmer's shrine (which is a choice purely based on efficiency). I would most likely run gladiatrix , the nature affinity though. Swift is a pretty big advantage over the shadow affinity. An essentially 2-3 card t2 is pretty optimal for rPvE 9 so either Gladi Lavafield or Phoenix Embalmers. Alternatively Windhunter, Ravage, Lavafield. Bandit Stalker is just a terrible card tbh.
T3 basically same rules apply though to a lesser extent because you have access to cards that can have a lot of value even in t4. So the obvious unit to play here is Soulhunter and then add in Frenetic Assault, Infect and Soulshatter. To round out the combo you cna do Cultist Master + Furnace of Flesh to get back the power from spells used or Shrine of War for the same effect. I would definetly run Shrine of War for Bandits because it is a very spell intensive deck and therefor needs a lot of void manipulation in t3 and t4. CM + FoF are not necassarily able to provide enough of that, especially if you dont use the CM ability on cooldown.
For t4 I'd run Bloodhorn, either affinity is fine, and maybe Rifle Cultists with Offering in addition to both cluster explosions and Earthshaker if you chose the shadow affinity Bloodhorn. In this deck it is CRUCIAL to use Bloodhorn's Stampede. It is your main healing tool.
That would leave me with these cards for your deck:
T1:
Forsaken or Nox-Trooper (Phase Tower)
Soul Splicer (Skeleton Warriors or Dread Charger)
Motivate
T2:
Embalmer'S Shrine (LavaField)
Shadow Phoenix (Gladiatrix)
Resource Booster
T3:
Soulhunter
Furnace of Flesh
Cultist Master
Shrine of War
Frenetic Assault (nature affinity)
Infect
Soulshatter
T4:
Bloodhorn (shadow affinity)
Cluster Explosion (fire affinity)
Cluster Explosion (shadow affinity)
Earthshaker
Rifle Cultists
Offering (nature affinity)

That leaves you with 19 cards currently. The other card is up to you really. Some possibilities would be:
Inferno,
an additional t1/t2 card (like Gladiatrix in a Phoenix deck to deal with Lost Vigils f.e.),
Nether warp for the utility it can provide.
Blood Healing, if you have issues with healing alot (on Infect Crawlers or a Bloodhorn that you offer afterwards)
Disenchant (nature affinity)
Unholy Hero

For any questions you might have about this deck, feel free to ask

To be honest it doesn't really matter why it is so difficult to implement. If it really is so much work then this feature will probably not be worth the time for quite a while.