Pulsemage Advocate does not have summoning sickness.
Steps
Activate Altar of Dementia by sacrificing Great Oak Guardian, causing target opponent to mill four cards.
Activate Pulsemage Advocate by tapping it and putting three cards from an opponent's graveyard into their owner's hand, returning Great Oak Guardian from your graveyard to the battlefield.
Great Oak Guardian enters the battlefield, untapping all creatures and giving creatures you control +2/+2 until end of turn.