I’ve added By-Ulf’s boast ailment to the status effects guide:
Note that the ailment shield buff from a flank like Gato can block the boast ailment, and also block the resulting undispellable ailments that follow after Boast’s turn count expires.
So if you use By-Ulf as a tank in 3* raid tournaments, I recommend flanking him with Gato to help prevent his downside from negatively impacting him after 4 turns.