Cocytus Showdown can be farmed on Hard or Very Hard fairly easily for newer players, allowing them to easily accumulate event items. For newer Water players, unless you happened to draw a good Water ATK summon, getting at least 1 copy of Cocytus to use as your main summon will be very beneficial.
None of the event weapons are worth spending resources on, so you can use them if you happen to get them, but don't use items to buy them or spend fodder skilling them up. Vergilius is worth holding on to as a potential Water weapon for Spartan/Holy Saber, if you might play Water down the line. As with all showdowns, you can buy 5 Half Elixirs per month, and you should be able to get enough materials to buy all 5 each month while using less than 375 AP, so it's worth spending the time to farm materials.
Simplified drop table
Disclaimer This section is still being written; it may be wildly inaccurate or missing significant data. If you can help please feel free to edit it but please write edit summaries, preferably with sources.
Water damage to all allies. Cocytus gains BlindedAttacks have a slight chance to miss , ATK UpATK is boosted , and TA UpTriple attack rate is boosted .
AntenorN
Water damage to one ally and inflict CursedCan't be healed for 3 turns.
Water damage to one ally and heal own HP based on damage dealt.
Depthless Despair
30
470,000
◇◇
Yes
Special Attacks
Triggers and Notes
Long for Embrace
3-hit Water damage to random allies.
Streaks of Bluesilver
Medium Water damage to all allies and gain RepelTaking less DMG and reflects attacks Damage Cut reducing damage taken and reflecting attacks. Warning! Does not work for Plain Damage! (500 damage).
Wave 2
Jiao
50
2,500,000
◇◇◇
Yes
Special Attacks
Triggers and Notes
Bite to Death
Large Water damage to one ally and inflict Skill SealedCan't use skills and C.A. SealedCan't use charge attack for 5 turns.
Wave 3
Yeti
40
1,200,000
◇◇◇◇
No
Silt Ray
40
850,000
◇
No
Boss
Cocytus
75
9,700,000
◇◇◇
Yes
Special Attacks
Triggers and Notes
AntenorN
Water damage to one ally and inflict CursedCan't be healed for 3 turns.
TromeaNOD
Large Water damage to 2 allies and inflict C.A. SealedCan't use charge attack for 3 turns.
Judecca - The 4th CircleTR
Water damage to all allies. Cocytus gains BlindedAttacks have a slight chance to miss , ATK UpATK is boosted , DA UpDouble attack rate is boosted , and TA UpTriple attack rate is boosted .
Tromea - The 3rd CircleTR
Large Water damage to 2 allies and inflict C.A. SealedCan't use charge attack for 3 turns. Removes all buffs and debuffs from Cocytus.
High innate double attack rate.
50% Trigger
Casts Judecca - The 4th Circle. Consumes all charge diamonds.