How can I say someone is doing their work in a half-assed manner?
敷衍 as an adjective or adverb can be used to describe a perfunctory attitude; consider the phrase 敷衍了事 (lit. to skimp through so as to finish something), or the idiom 敷衍塞責. Synonyms include 馬虎 and 草率, which are slightly more colloquial. As in, for example, 做事馬虎, 辦事草率. Or even 馬馬虎虎, but seldom 草草率率, nor 敷敷衍衍. 搪塞 also works, but only as a verb. If you want to comment on the lack of quality of someone's work as a result of their attitude, you may want to use these.

全力以赴, 盡力而為, 出盡全力, 用盡全力 and the like all describe the opposite; you may accuse (or rather, encourage) someone for not making their utmost effort by negation, such as adding 沒有 before these phrases. Or even 付出100%的努力 (lit. to pay 100% of one's effort, as stated in your question) is perfectly understandable.