Contrary is one of the most disruptive abilities in Pokémon Champions. It inverts all stat changes — boosts become drops, drops become boosts. On Mega Staraptor, it transforms Close Combat from a move with a self-punishing drawback into a move that actively makes Staraptor harder to kill.
Close Combat deals massive damage and would normally drop the user's Defence and Special Defence by one stage each. Under Contrary, those drops flip to +1 Defence and +1 Special Defence. Every Close Combat Staraptor lands makes it simultaneously more dangerous offensively and more resilient defensively.
In practice, this means taking Mega Staraptor down requires KOing it before it has had a chance to accumulate defensive boosts — because once it has even one Close Combat under its belt, priority moves and chip damage that would normally finish it off become unreliable. After two or three uses, neutral attacks that would clean up a weakened attacker are no longer guaranteed to do so.
The Flying/Normal typing does have weaknesses. Rock and Electric moves hit Super Effective, and Staraptor's base Special Defence is not exceptional before the Contrary boosts kick in. Speedy Rock-type attackers or Electric moves before Staraptor gets a turn in are the cleanest answers.
Team structures that support Mega Staraptor typically pair it with speed control — either Tailwind to guarantee it moves first, or a screen setter to reduce incoming damage on the critical early turns. Once Staraptor has one or two stat boosts, those supports become less necessary.
Contra plays like this are what makes M-B a more complex format than M-A. If you want to see full type matchups, the type chart shows exactly what threatens Staraptor and what it resists.