1) You can farm it via the Angel Halo (Quest -> Special (Scroll to the end), (check https://twitter.com/granbluefantasy for the tweet where you see a '★' beside a timing (timings are in JST and varies daily), under each group section(to know which group you belong to, check your Granblue in game profile (Menu (top right) -> profile) look for the Group just above your HP stat.). Reason behind that is that during the timing with a '★' you get exceptional amount of loot compared to usual. So people usually farm during that time and last for an hour.)

Or leech a Proto Bahamut Raid (100% drop iirc) (use https://tweetdeck.twitter.com/ and search for Lv100 プロトバハムート)

2) Most people get Dark Fencer's Miserable Mist (Atk and Def debuff), Weapon Master's Rage III (30% Atk up), Sidewinder's Arrow Rain III (dmg to all foes and atk debuff), Hawkeye's Bounty Hunter III (Boost to loot Drop), Cleric's Clarity (removes 1 debuff) and Bishop's Dispel (Remove a foe's buff effect) These are common popular ones.

3) I cleared with Yodarha, Percical, Andira and Lady Grey and Superstar (iirc) with Grande Order summon.

4) Here: https://www.reddit.com/r/Granblue_en/comments/4rxqg6/when_to_decide_if_an_event_is_worth_it/