To express surprise with disappointment, if 卧槽 is vulgar, is 苦也 considered a decent alternative? Many people say 卧槽 when something unpleasant happens surprisingly. However, this word is considered as vulgar. So how to express such feelings decently without using offensive words? Is 苦也 a good choice?

Compare the following:


  ……只觉重门叠户之内，如处隐伏甲兵。李思永叫声卧槽，流星锤舞得呼呼风响，把一扇门板打碎，……


and 


  ……只觉重门叠户之内，如处隐伏甲兵。李思永叫声苦也，流星锤舞得呼呼风响，把一扇门板打碎，……


Are they basically mean the same thing? The second one seems more polite to me.
卧槽 (我操) is mostly used as an exclamation, It can be any phrase as long as it expresses the emotion the speaker intended to express. Like 'rage, surprise, displease' and so on.

Some examples:

可惡 (rage/ anger/ displease)

可怒也 (rage/ anger/ displease)

糟糕 (surprise, worry, fear)

不好; 不妙(surprise, worry)

管他 (dismissive/ self-abandon )

豈有此理 (anger, displease)

弊了 (surprise, worry, fear)

In your example, either 可怒也 or 可惡 would be a good choice


  ……只觉重门叠户之内，如处隐伏甲兵。李思永叫声可惡，流星锤舞得呼呼风响，把一扇门板打碎，…… (he was angry about being ambushed)


But other exclamations can be chosen too, depend on what emotion the author wanted the character to express


  苦也 express 'despair' or 'disparate', it does fit in the example too, but the emotion would be different from other exclamations I listed