However, the explosion that Rainbow Dash did was viewed at a smaller distance away, and it only blew up the (already weakened) barn. Meanwhile, Twilight's was preceded by a giant magical blast and was a considerable distance away, as well as ravage the landscape. In summary, Rainbow Dash vs Tirek would not be anywhere near the level of the fight in the show.

"All the magic in Equestria"
No. Just the magic from the known alicorns. If it was all then Tirek wouldn't even have gotten powerful in the first place.