There were only 2 major resources that I really used to get a 97 in 2U
thsconline for full selective/private school past papers. Past papers are the key to success in 2U, and each school's papers have distinct differences. Sydney Grammar papers, for example, have fairly difficult geometry problems. A wide variety of past papers will help you in answering exam questions, and identifying where you need to improve
khan academy for visualisation. Half the time in 2U, you can get an approximate answer to the question just by visualising the process. Visualisation is a key process in efficiently checking your answers, especially in differentiation and questions involving curve manipulation.
Do a past paper, see what you struggle with, and attempt to visualise it. Repeat.