There was very little that can be called unrealistic with the final matchup; that's not the real issue. The real problem is Iris' semi-final matchup, because THAT was, in some ways, even worse than Pikachu vs Regice. Axew had no on-screen training outside of perfecting Dragon Rage -which only makes learning Outrage at the most convenient time right at the spot even less deserving- and yet still comes out on top without Iris even giving the command. Completely left field just like Drilbur's 99-win streak. Axew ALONE won that battle; Iris did nothing to win.
Not to mention Iris didn't deserve to win the tournament. Iris did very little (if anything at all) to make her deserve to win any sort of tournament. The whole "maybe Iris did lots of off-screen training" excuse doesn't fly. The writers have to do their part to SHOW the characters' growth or it doesn't count.
Iris single-handedly ruined what had huge potential to be an epic tournament -despite the overused running gags- with some good battles and great rivals to boot.