Tenzin was Avatar Aang's oldest son, and had a much better relationship with his father than Aang's other two kids. Zaheer did not stand any chance of winning that fight, as Tenzin was visibly much more skilled than him.
Keeping this in view, can Tenzin beat Zaheer?
If you'll watch that battle again, you'll see that tenzin is actually winning the battle against Zaheer. And that makes perfect sense. Tenzin is an airbending master, son and student of the legend himself, Avatar Aang.
Similarly, is Tenzin stronger than Aang? Versus the EOS Aang from TLA, I'd say Tenzin is stronger. But against the adult Aang we see in TLoK, I'd say Aang is much stronger than Tenzin. Well Aang has definitively better and much more feats than Tenzin but Tenzin has clearly a better mentality for a fighter.
One may also ask, who is stronger Tenzin or Zaheer?
Tenzin still wins, because we're still talking about a Master vs. a skilled enthusiast. Zaheer can let go his earthly tether all he wants, but it'll make no difference in the end. By the way, if you're looking for "who would wins" like this, check out r/AvatarVsBattles.
Why is Zaheer so good at Airbending?
Zaheer was likely a master martial artist before he became a bender. All bending is based on a form of martial arts and there is more than one viable form so Zaheer could have developed his own style.
