By training it. The way one might train to wield a weapon.

( It's the only way. Their minds are too vulnerable to attack, and have been long before this, like the lighthouse. Before that with the practices of the Attaryl. Possession. The thrall of mermaids and goddess knows what else. Magic wards only go so far in this world. )

Wrath is well-versed in mental barriers, although mainly from our world.

( She knows the two of them are close, and Wrath is good target practice. She first learned this the hard way, but it's also how Emilia was ultimately able to resist demonic influence. Why she was able to fight the lighthouse's influence. )