Do I need to do act 3 in order to get 100% completion?

I'm currently 8 hours into my second playthrough, I'm aiming for the "100% in less than 30 hours" achievement and I have all powers but still need to do lots of clean up and that will probably take 12 hours. I was wondering, I've seen people say act 3 can get pretty lengthy with bosses but I'm scared I won't make it before 30 hours as I've never been to act 3 before. But I've also read online that you need to get 100% IN act 2 to get the trophy to pop up, and if that were true then it would be a lot easier for me to get it. Please, let me know!