The hardest boss in this Singularity is Solomon and the Four Devoting Pillars, at the 13th chapter, fifth battle.
If you have been powerleveling, the boss isn't that hard. But for those who haven't...
Tips and Tricks
Solomon is a Grand Caster.
He deals extra damage towards Knight classes (Saber, Lancer and Archer) and takes extra damage from Cavalry classes (Rider, Caster, Assassin and Berserker).
However, he does not take decreased damage against Knight classes or deal decreased damage against Cavalry classes.
He gets around 3 actions per turn.
He has a tendency to spam the skill Browse, which inflicts Burn and decreases defense, the latter can stack.
His other skill is The Time of Despair has Come, which will not only debuffs your party's Critical Star drop rate and decreases their NP bar, but also increases his own Critical and NP power. He doesn't spam it as often as Browse.
All of his attacks deal damage to all allies, so taunting does not work here. The same goes to his Noble Phantasm/Skill; Incineration Ritual: Beleth. Combined with his tendency to decrease defense and raising his NP power, this can be dangerous if left untended.
Burn can kill a Servant hiding behind Evade/Invincible, so either heal or remove the debuff whenever possible and said Servant is in extremely low HP
Riders, Casters, Assassins or Rulers (at this point, only Jeanne d’Arc) are recommended since they do not take extra damage from his AOE attacks.
Servants that can increase defense, particularly to the whole party are recommended (prominently, Zhuge Liang)
Servants that can remove debuffs are recommended, even moreso if it's affecting all allies (Saint Martha). Try to find Cavalry-class Servants with such skills.
Mashu Kyrielight work really well here due to being unaffected with double damage based on class and being able to raise the defense of the allies on demand.
David can work in here despite taking extra damage (being an Archer), since he possesses an all-allies Evade and heal at the same time, which can help stave off Solomon's party-wide attacks (especially if his only uses one action to attack)