Right off the bat, take Che Goat to R2. She is one of the best counters for 8.2 boss and will help you immensely with quite a few other fights as well.
I become valiant last week just exploring act 8 and competing act 9.1 with some paths more for the alpha 4, exploring the act 8 is really boring but easy way to get a r3 gem, 8.3 and 8.4 gives very good rewards 8.1 and 8.2 do not. Start with 8.3 or 8.4, to get more resourses to rank up champs, after that will be easier to explore the rest.