Without Sparing, Spare No
い adjective
Remember how you learned that 惜しむ means "to be sparing?" Well if you make that negative with ない, then you're doing it without sparing anything instead.
おしまない
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You learned this reading already, so you should be able to read this already too!
目的を成し遂げるには、努力を惜しまないことが大切だと思います。
I think it’s important to spare no effort in fulfilling your goals.