EDIT: As of today's patch (May 20th, 2021) the infallible trophy is no longer obtainable from this method.
This exploit may be patched at any point but with yesterday's season 4.5 patch (May 13th, 2021) it now includes the option of custom lobbies. To my knowledge only this trophy will pop with this method.
You will need someone to start a lobby and share the code with at least 3 others. Decide on an order for who will get the trophy first. Once you have 4 you start a game of Hex Trials. (Any more than 4 will require you to play two rounds per crown)
The winner will stay at the top and the other 3 will just jump directly in the slime. After 5 wins in a row the winning player should close the game as soon as they see the last podium screen. Once they reload the game the trophy should pop. Then they should rejoin the lobby to help the others.
The host of the lobby should be the last person so that a single lobby can be used throughout. After 20 rounds all should have the trophy.