2.1 Common Colloquial Phrases & Idioms:
"Chip away at": To make gradual progress on something. Example: "I chip away at my vocabulary list every day."([Reddit][2])
"Plug away": To continue working hard at something. Example: "She plugs away at her grammar exercises each evening."([Reddit][3])
"A necessary evil": Something unpleasant but needed. Example: "Studying grammar rules is a necessary evil."([維基百科][4], [Reddit][2])