What about this?
 

I don't understand the meaning of a part of sentence when it is seen in the global context of the previous part ... Nagoyankee, please tell me what it could mean.
First of all I'll tell you the meaning of the previous part as I understood it(where we were arrived to see it).
" ... unfortunately I am crouched inside the air raid shelter refuge and on my knees is opened only an illustrated book for children. I, depending only on this illustrated book,"
論断せざるを得ないである
論断 is "conclusion" but "I have no other choice that to conclude", is it possible?
It seems to me that there is something that I don't understand. I'm wondering if Dazai wants to tell the reader that he must conclude this matter about oni, kisai and bungaku no oni. If it isn't so, what does that " 論断せざるを得ないである" mean?

I'm no Nagoyankee but I'll try to help. I can't comment on the exact sentence unless you post the full original Japanese sentence.

せざるを得ない is an expression meaning "having no choice but to".
ざる is a old form of ない. する's ざる form is　せざる. Other verbs follow the usual rule, e.g. 走らざるを得ない I had no choice but to run.

You appear to be very confused but the answer to your question lies in the immediately preceding sentence. One needs not go back any further.

論断せざるを得ないである = "I've no choice but to reach a conclusion"

「このへんで、日本百科辞典でも、ちよつと覗いてみる と、私もたちまち老幼婦女子の尊敬の的たる博学の士に 一変して、（世の物識りといふものは、たいていそんな ものである）しさいらしい顔をして、鬼に就いて縷々千 万言を開陳できるのでもあらうが、生憎と私は壕の中に しやがんで、さうして膝の上には、子供の絵本が一冊ひ ろげられてあるきりなのである。私は、ただこの絵本の 絵に依つて、論断せざるを得ないのである。」

If you 日本百科辞典でも、ちよつと覗いてみると, you will 博学の士に一変.
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Therefore, you will be able to 鬼に就いて縷々千万言を開陳できる.
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You are now , however, 壕の中.
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In the 壕の中, you don't have the luxury of reading the 日本百科辞典.
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All that is available to you is the 子供の絵本.
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This is why you have to reach a conclusion from the content of the 子供の絵本, no matter how ludicrous that is. Needless to say, the conclusion is about
all kinds of oni that exist.

I must remind you that the author keeps telling understated jokes. If a reader takes one of them seriously, he will be stuck.

But he is really telling too many understated jokes. Well, I've understood now.
Thank you very much (as always).