Eradicator Valkyrie

Eradicator Valkyrie can't be the target of abilities whose source is a planeswalker an opponent controls.

As the boast ability resolves, first the next opponent in turn order chooses a creature or planeswalker they control. Then each other opponent in turn order chooses a creature or planeswalker they control, knowing the choices made before their choice. Then all chosen creatures and planeswalkers are sacrificed at the same time.

If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.