Dungeon Three (Fire Dungeon)
Head to the third dungeon, which is located in the volcano on the world map. You'll have to complete the first and second dungeons to unlock this dungeon. This volcano-themed dungeon is the only location in the world of "Treasure Abyss" that holds the Flame Crest.
Dungeon Three, 4F
Fight your way down to the fourth floor (4F) of the third dungeon. Search this entire level for the chest and open it. Because there is only one chest on this floor, it may take a few minutes to find. Because this item is randomly seeded in the chest on 4F, you may need to exit this dungeon, then re-enter and go back down to 4F to open the chest again. Eventually, you'll get the Flame Crest instead of a candle, which is normally inside this chest.
Slot Machine
At the beginning of each game of "Treasure Abyss," you can spin the reels once on the slot machine for a chance to get a rare item or weapon. The chance of getting a Flame Crest is exceedingly small, but it is possible, since the Flame Crest isn't a completely unique item; many players who play Dungeon Three will end up with one.
Trading
In the in-game store, you can request from and offer items to other players on your buddy list. Many players may be willing to trade some types of crests, which provide players with elemental protection that will help them survive in certain kinds of environments. This is more likely to succeed than the slot but less likely than retrieving it from Dungeon Three, 4F.