Onigashima Raid Boss Guide

Short Summary

The ascension to the summit is blocked by Gates with Oni raid bosses
These raid bosses have five escalating difficulties ranging from newbie-friendly to veteran level.
Unlike Rashomon, the points ladder is less steep and does not necessarily require clearing the hardest raid node to finish in time.
In fact, the points ladder can be cleared comfortably by Masters who only do the 1 BP and 2 BP raids.
Each gate is guarded by an Oni boss accompanied with a Servant of a different class.
The peak itself is guarded by a Berserker Boss with two mobs, similar to Ibaraki.
After the main quest has been cleared, each gate needs to be cleared anew and the Oni will receive a different Servant to fight alongside with.

Buy out copies of Golden Sumo as soon as possible and LB it.
Consider feeding your MLB copy of Golden Sumo with additional experience to upgrade its attack stat. It’s also an extremely powerful Craft Essence and a mainstay for many Masters even after the event.
Equip multiple copies of Hot Springs of the Moon in the backline and on support Servants.
Japanese Servants receive an additional bonus damage.

All Servants also receive an additional damage bonus depending on the difficulty of the raid quest that is selected.
However, the more bonus damage Masters stack from CEs and as higher difficulty raid quests are selected, the less relative impact this Japanese damage boost has.
Each raid boss (outside of the fourth one) is vulnerable to exploiting class advantage and their NP is not immediately lethal. This makes surviving much easier than in Rashomon.
Each gate is guarded by an Oni with a resistance to a particular card type.

A bean consumable can be used to nullify this protection - highly useful for struggling Masters or those wishing to speed up their clear time. These consumables can be traded for a minor amount of QP after the event ends, however.
However, rather than nullify a protection, it is often more effective to use a bean of a different card type to boost performance instead.
Each Oni has extra vulnerable traits: They’re male and Demonic.
The key to success is being able to stack offensive buffs properly and to exploit class advantage or trait advantages without having to deal with the card type demerit preferably. Since survival is easier than during Rashomon, we can bring a lot more Servants with only offensive buffs.
However, try to bring at least one Servant that can sustain the team with healing or defensive buffs.It’s best to keep the Servant with Golden Sumo equipped alive as long as possible.
Masters with a powerful (class-advantaged) Servants of their own should borrow a support, while Masters without a good DPS for the current raid should borrow the ideal DPS servant from a friend.
Always abuse the first wave for generating NP gauge, use Overkill and Arts chains.

You will keep your damage done even if you die to the raid boss unlike in Rashomon.
Taunts aren’t that useful for these raids, all raid bosses have an AoE NP.
The mobs in raids can drop different materials based on the difficulty and don’t just drop currency alone.

For newer Masters who are unfamiliar with the hardest content in the game, or whether you simply need a refresher, the Challenge Quest Compendium goes into detail about how the best teams are formed, how the enemy AI generally works and what to look out for in a fight.
The raid content of Onigashima is a good training ground for the upcoming singularity, which is going to demand the utmost out of every Master.
If you are struggling with any of these raids, this guide will be a good place to get some ideas.