magarena/release/Magarena/scripts/Diminish.groovy

def PT = new MagicStatic(MagicLayer.SetPT, MagicStatic.UntilEOT) {
    @Override
    public void modPowerToughness(final MagicPermanent source, final MagicPermanent permanent, final MagicPowerToughness pt) {
        pt.set(1,1);
    }
}

[
    new MagicSpellCardEvent() {
        @Override
        public MagicEvent getEvent(final MagicCardOnStack cardOnStack,final MagicPayedCost payedCost) {
            return new MagicEvent(
                cardOnStack,
                MagicTargetChoice.TARGET_CREATURE,
                new MagicDestroyTargetPicker(true),
                this,
                "Target creature\$ becomes 1/1 until end of turn."
            );
        }
        @Override
        public void executeEvent(final MagicGame game, final MagicEvent event) {
            event.processTargetPermanent(game, {
                MagicPermanent creature ->
                    game.doAction(new MagicBecomesCreatureAction(creature,PT));
            } as MagicPermanentAction);
        }
    }
]