Well, that giant crab challenge quest was a bit of a pain, I don't have a friend with Angra on their support and even if they did I don't really have a good party setup to utilize his NP to its fullest potential.
SO I just took Mashu/Euryale/Merlin and gradually chipped it down with flat damage. Core of the Goddess EX and Covering Fire skill means every hit from Euryale was going for 700~800 damage so not too bad. It took me 62 turns in all to finish it off, making it even longer than Salter's memorial quest, but also far less dangerous.
In hindsight it might have been better to take Mashu/Hans/Waver instead because Waver's attack buff also grants flat damage, which would have meant everybody can actually hurt the crab. And the curse from his NP would have been a small but welcome bonus too.
Edit: Covering Fire is a CE, not a skill :V that's what i meant