Dies to Removal

What is the latest during combat you can cast an instant on Akroan Crusader and still be able to attack with the hastey solider token? Thanks in advance for your help.

You can cast it during the "Beginning of Combat" step, but no later. The next step is "Declare Attackers" and you're required to declare your attackers in full at the start of that step, before any player receives priority.

The Turn Structure wiki page is helpful for things like this.