Maybe you should have learned the forms of the aorist passive before attempting this sentence, JM!
νῑϰηθείς is an aorist passive participle, masculine singular nominative to modify the subject of the sentence, "you." The μὴ makes it negative. It is used as the protasis of a contrary-to-fact condition: "Not having been beaten," i.e. "If you hadn't been beaten."
And ἠξιώθης is aorist passive indicative (with ἄν, which makes it the apodosis of a condition) of ἀξιόω: "you would have been considered worthy."
Finally, how could ἄθλου be anything but genitive singular?