The Special Boss
After the 7th week on your 2nd playthrough, If you finished all of Taiga's request, she
will give you a message that the arena is dangerous. There you will find a Special Boss
fight at the end of the floor.
Saber's Skill Thunderous Applause
Saber's skill, Artillery Applause can be upgraded by using around 100-200 items during
your run through the arena. After you used enough items you'd be able to speak with
saber in your private room. If eligible, she'll be talking about wasteful spending and you'd
get the confirmation that Artillery Applause have been upgraded to Thunderous
Applause. It will cost 60mp instead of 40mp.
Caster's Skill Mantra: Merciful Sky
To upgrade Caster's Mantra: Bestial Sky to Merciful Sky, you must kill 5 of the Nephilim
enemies throughout the game. Afterwards, you can initiate a conversation in your
Private Room which will then upgrade Bestial Sky to Merciful Sky. Merciful Sky increases
the next spell's power more than Bestial Sky. It also increases the MP cost from 30 to 60.