Well lets see...
I havent seen the previous parts, but by watching this movie I think I already understand what this is about..
Now, one thing I have to comment on first is the use of your dialog, You hardly used any dots or commas at all, which made it hard to know exactly where the break in the texts, where as you used a whole shitload of exclamation marks, it could have been enough with one, even though they are acting serious.
The story? Its a bit meh, I guess it could work but the lines you used were some of the most overused and cheesy ones I have ever seen, like Mario saying how he always keep having bad dreams. Also, I would understand Mewtwo talking cause he talks, but Pikachu and Jigglypuff? And to top it off Pikachu had to act all badass sounding as he is some god of lightning or something.
One thing I did like about this alot was the fights. They were quite top notch. I somewhat expected that Crazy Hand would get into the fight aswell. The scene with Snake appearing was a bit... well... I dunno. I guess it was just to simple.
Overall you did quite a good job on this, but the ending went rather weird, as you just showed Sonic being the next newcomer, and it then faded out into the credits. You could have made this into something much better, like having him appear into the fight out of nothing.
But the flash was quite good actually. Wasnt the best, but still, not bad at all.