Yes, I agree with TOMO@-SAN
It's interesting to learn 四字熟語
Actually they are difficult to use for native Japanese, sometimes.
「悪戦苦闘」 is useful in writing and conversation, i think.
犬のしつけに悪戦苦闘してるよ( ̄~ ̄;)
I'm trying to make my dog disciplined, but it's very hard!・・・i don't know if it's correct English・・・
and
I use 「暗中模索」in writing.
I rarely use it in conversation.
for example,
彼は自分の将来について暗中模索しているようなので、少し見守ってあげましょう。
He seems to grope for his own future , let's wait and see.
sorry for my poor English,
but i hope they are helpful.
Thanks for all the replies. It is really helpful to hear which one's are actually used in conversation, which ones are written, and which ones aren't really used.