This question came up on chat.
In this phrase:
Can the comma be replaced by を? If not, is there a better particle?
頑張る is listed as a 自動詞 in both of my dictionaries (岩波 and 集英社). There are plenty of Google results for を頑張る though, including in phrases like TOEICを頑張る. I've also found some related discussion, though it didn't answer the question.
Possibly related: Are there verbs that are neither intransitive nor transitive?