Showdown is a good way to pick up some extra scroll and item pieces. The bosses rotate so you get a new one every day. You get 2 free chances to beat the boss per day and you can't buy more with crystals. If you fail to beat the boss you get to try again, and can change to a lower difficulty if needed.
Possibly the most difficult boss. Turn off Auto Attack, Bring Garmel, Jedy Cat, Sally and two good Damage Dealers (I use Richen and Lilith). Only use your damage dealers' special attacks, heal with Sally when necessary, save Jedy Cat's and Garmel's skills. Then when Id's health bar just drops below his nametag in length, just before he heals, pop Garmel's skill, then nuke him with Jedy Cat.Opinion by AzakaZero
This guy is pretty straightforward. The three recommended heroes all have attacks that are ice-based, which of course a fire demon like Ignitus is sensitive to. So bring Richen, Emma and Yuffy, then Sally for some heals after the boss does his area attack (he is also susceptible to Sally's charm ability). For the fifth member I bring Karin to increase overall survivability.Opinion by AzakaZero
This guy is the easiest boss of them all, as long as you can bring an all-male team to the fight. You can kill him on Hellfire difficulty with maxed 4-star male units. Moik, Jedy Cat, Richen, Rath and Ethan makes a great team for it, but there are other options.Opinion by AzakaZero
I don't really know what this old geezer's weakness is, it says he's old but I'm not sure how that translates into game mechanics. He's not that hard to beat, though. The game suggests Rath, Lysterine, Rudy and Yuffy but I find that he goes down just fine using only Rath and Yuffy. This team destroys him on Hellfire difficulty autobattle: Rath 4★ 80, Yuffy 4★ 60, Lilith 5★ 90, Karin 5★ 90, Sally 5★ 90. Methuselah can be charmed by Sally, by the way, and be petrified by Lilith.Opinion by AzakaZero