Some questions about Xerath

Hi, i'll go straight - So, is building roa first good option? we kinda lack damage but we wont get oneshoted by everyone. What's the easiest way to get mastery 7lvl? its pretty hard for me to get it on midlane even though ive got good kda and cs. How should my build look like when im against heavy ap damage and cc? What should i build when i fall behind? Is 24/7 poking with Q a good option? In what team comps should i pick xerath? Ty for the replies, regards.