Find a native English speaker to provide the "correct" answers.
From Chinese 完全
hoàn toàn đồng ý =完全同意？
Why was 'completely' cách hoàn toàn before, but now it's just hoàn toàn?
you would use "một cách" when you need to change a noun or an adjective into an adverb. "hoàn toàn" by itself can be an adverb thus doesn't require it.
ps. "một cách X" not just "cách X".
Alert for course updaters - "agree completely" should also be accepted. It is perfectly acceptable English.