Congrate for the second interview!
-- Dress nicely, usual business dress will be OK;
-- Many smile from begining till end;
-- Don't be nurvers, they asking you for a lunch interview meant they like you so far;
-- It should be a relaxed lunch with any kind of topic, so do not have to be afraid of, otherwise you will be too careful to talk.
-- On the other hand, this is still part of the interview, therefore, you still focus on selling youself in a natural way, but not push too hard.
-- Show them you are a person easy to be with, you respect them, you are not too picky;
-- Order some food which is easier to handle, well cooked fish with rice will be fine;
-- The people in higher management level usually will not ask you deep technical questions, but will ask something like why you like this company, what is the reason you switch job previously (to figure how you bahaved in each position), and what is the criteria that you chose the new company...
-- Relax please, show them the natual YOU. No matter what, this will be the 唯一的永远.
Thanks!