First, your "summary table" is basically a Players table and it would be best to treat it as such - for example by including the player's names in it (once) instead of repeating them in the Attempts table.
There are several ways you can summarize the Attempts table. First and foremost, you can produce a summary report of the table itself, summarized by PlayerID and by Level. To exclude scores below a certain threshold, perform a find as a first step of producing the report.
Another option is to use filtered portals on the Players layout, one portal per Level (or, if you prefer, use a single portal and a global field to select the level being shown).
Yet another way is to add a Levels table, and show the results in a portal to that table (though this is a little difficult to describe properly in a few short sentences). It's also not quite clear if you want to show all attempts above the threshold,or only the top score - this method would only show the latter.
The reason why your calculation cannot work is because the expression:
PLAYER_ATTEMPTS_TB::Game_Level
is evaluated using data from the first related record.