Before these two tests:

  Test.assertDeepEquals(Goku.battle(9), "An intense fight", "Your warrior picked a fight with a relatively strong enemy. Nothing he can't hold his/er own to. Provide the appropiate response")
    Test.assertDeepEquals(Goku.battle(14), "You've been defeated", "Your warrior picked a fight he/she can't win.")

The warrior level is actually should be 4, not 3 as seen in the tests below.

    // Before
    Test.assertDeepEquals(Goku.level(), 4, "Your warrior should now be LV 4")
    Test.assertDeepEquals(Goku.experience(), 400, "Your warrior's experience should be relative to his level")
    // Now
    Test.assertDeepEquals(Goku.battle(9), "An intense fight", "Your warrior picked a fight with a relatively strong enemy. Nothing he can't hold his/er own to. Provide the appropiate response")
    Test.assertDeepEquals(Goku.battle(14), "You've been defeated", "Your warrior picked a fight he/she can't win.")

With the current total experience of 400. So, gained exp is 20 * (9-4) * (9-4) = 20 * 25 which is 500. So, the new level should be 9 (because 400 + 500).

The warrior is 5 level from 14 and is lower in rank (the warrior is Pushover, meanwhile the enemy is Novice).

Test cases for JavaScript aren't following instructions given. Exp given for battles is ( 20 * diff * diff ) however leads to errors in testing. By the time Warrior() is level 3, fights level 9, gains 720 exp, ends up as level 11, fails test because next fight is level 14 and test says the fight is supposed to be a defeat.