Let's start with the vision: an epic battle from Twilight's perspective, but also angled so you could see the opponent, with the elements going crazy around them. That was delivered properly, with poses, expressions, and effects.
Cannot say this is very original, though; for better or worse, there are a bit too many picture of Twi and Trix killing each other. However, the addition of rain is VERY original.
Now, the technique. Everything aside from water is good. The magic is... well... magic, so it can look like anything, as long as it is well-drawn. Poses are perfect. Very often people fail at drawing 3-dimensional poni, but you succeeded. Wet manes also look good. However... water... it does not look like water, and more like ice or sand. Water does not splash properly, splashes are too angular, and the waves are too round. Work on your water.
Impact = vision + originality + technique.