What's the likely reason?

Near the end of one of the last matches that I played in Mer Stadium this afternoon, an enemy scizor approached our goal-zone and though a teammate and I attacked him relentlessly for what must have been at least 15 seconds whilst we stood on our goal-zone, we were unable to reduce his hit-point bar to even a quarter because he appeared to be losing too few hit points and regaining too many (the relevant teammate played hoopla and I played metagross).

We finally defeated the enemy scizor when a third teammate joined the attack and even then, we struggled.

Was this the result of a glitch?

Are some matches programmed to be generally impossible to win?

Is this sort of resilience possible with some combination of held items, emblems, or both?