Well there may be some parts where mirror mode is not working, but specifically on Waterfall Quest, mirror mode adds a huge response delay (by default 1000ms) which would make it near impossible to complete on a level 3 without being 1 hit, its basically a 1 second lag on everything. Lowering it to 50ms may help, but I'm not sure what's the success rate on mirror mode when it comes to Waterfall Quest.