I assume you are asking where the dev room is. The secret development room is the room you access by using the tree five times in the park of the starting town. More details here. I think a NPC talks about it (or it's in the in-game hints).
Grandly went pretty hard for a while, the regen + Life Energy + Vortex made it nigh unkillable until enemy levels got stupidly high in DLC 2.
Edit: Shadoom actually has the perfect combo I want of super tanky + Lamenting Spite + Body Regen + Protect but I didn't figure that out till I finished the game. Grandly also has the regen which Shadoom doesn't but Shadoom has Soul Drain so who knows who wins.
Not sure if anyone has mentioned this already, but going to the second floor of the teahouse after buying the house just crashes the game? It makes 2 scenes unobtainable