I read the story in a paperback anthology published in the late 1960s or early 1970s. The protagonist is a human, a former soldier(?) who is now a champion "gladiator" in arena battles staged by insectoid invaders who have taken control of the Earth. His inner dialogue covers the backstory & history of the invasion; I remember something about the invaders being large mantis-like creatures with a very high-pitched call that can kill, easily defeating human armies. He is about to fight his last fight before being granted freedom by his alien captors and has a personal ritual where he envisions himself drawing power from the planet's core, strength from the rock, etc.
Suddenly he's told this has to be a partner fight, where he is chained to another human -- who turns out to be an inexperienced female gladiator. He is angry and she chides him for his use of the whistle-like device that lets him converse with their captors. The end of the story is along the lines of "let's show 'em what men and women can do together!" as they work out a plan of attack for the giant grub-like larva in a partially flooded arena. (They intend to trick it into breaking the chain that connects them so they can attack freely.)
The story closes with them ready and raring to go. I remember lots of details EXCEPT little things like title and author.