Because of the recent popularity poll, I noticed that the notable Japanese sources, thwiki and pixiv, use the "full" name 核熱造神ヒソウテンソク (かくねつぞうしんヒソウテンソク) to refer to hisou_tensoku, so maybe that's what we should use as well. It originates from the OST cover (http://www.tasofro.net/touhou123/cd.html)
I don't know, however, if 核熱造神 should be translated or kept as is, and if ヒソウテンソク needs a space or not. Unlike the game's title, it's written in katakana and may therefore be just a name = no spaces.
About the catfish boss:
From Pixiv :
ゲーム中で自称した「伝説の大ナマズ様」の呼称も使われる。
Our current tag, namazu, is just the Japanese word for "catfish". It needs the 大 to unambiguously mean the mythological creature which would make the tag oonamazu. However, that tag is currently used for an artist.
Updated