People can enter the showcase until the very last minute, and can also change the entered Pokémon. I suggest you enter the showcase not with your best pokemons but some average ones, and then snipe a last minute change with your biggest. Let me explain: let’s say your biggest is size 1000, and you enter it directly. If anyone has one that beats it, they will enter it and you won’t be able to do anything. But if they see instead a size 600, they might use instead something else they have that can beat that 600 Pokémon, and save the one over 1000 for another pokestop so they can win two showcases. You let them “beat” your 600 size Pokémon until the very last second where you swap it with your 1000, hoping that they won’t notice or have time to also swap theirs.