In brief: you want to run Apocalypse and Crom (one that's your's, one as a friend eidolon). If you don't have both, you have to rely on your friends having the one you don't (every battle, all the time). It's best to have both. While you can be choosy with Ragnaroks, you need a basic combo to spam Ultimates with that can you can form every time, no matter what the friend lottery gives you.