Because the last ability causes Monument to Perfection to lose all the abilities, activating it multiple times before the first activation resolves does not result in it having multiple instances of toxic 9
Oona also works particularly well with cards that check when creatures come into play, like Kindred Discovery and Dire Undercurrents 3 Tegwyll, Duke Of Splendor A true Faerie lord, Tegwyll, Duke of Splendor works to give your other Faeries a boost in power while ensuring that you can still gain advantage even after death
Its at its worst when there are lots of fast race matchups, like combo decks, burn decks, and very linear aggressive decks that arent usually removing your permanents (Goblins and Merfolk)
The most direct strategy involves giving bad gifts, cards that actively harm their new controller, such as Steel Golem, which prevents them from casting creature spells, or the notorious Illusions of Grandeur, which causes a 20-life loss when it leaves the battlefield