When Terapagos is hit by a damage-dealing move at full HP (that does not have no effect due to type matchups), it will always be not very effective.
Tera Shell has no effect on moves that deal direct damage or typeless damage (e.g., self-inflicted confusion damage, Struggle). However, in the case of the former, the message "It's not very effective..." is still displayed even though the damage was not actually halved.
