I'm really glad to see you've still been working on this, it's been a while! Now, on to the critique!
Right from the get-go it's an interesting change to see a panel done in a vertical format. This works really well to show off how unbelievably huge Mecha Pinkie is. The confetti and ribbons really help to establish a feeling of depth.
A few things that I think could be worked on:
I'm not sure how flexible Mecha-Pinkie is (she
is a robot, after all) but the pose looks pretty awkward. She seems to be straining her eyes unnecessarily far when she could just be moving her head to look at Twilight. But like I said, it's up to you to decide how flexible she's going to be and I could see it work either way.
The other thing that seems a little odd to me is how oblivious many of the characters seem to be to the giant robot right behind them. I've always really liked that you put a lot of little details into your work for your readers to find, but in this case I think they detract from how shocking and amazing Mecha Pinkie's appearance should be. This is the first time we're seeing Mecha Pinkie out in the open and in her element, and I wish the panel had focused more on that.
Of course, pairing this with the previous panel I understand that you might be using this to establish a nightmarish quality to the story. From Twilight's perspective: "Where did this party come from? Where's Pinkie Pie? Why am I the only one noticing this giant robot?" If you decide to go this route, as your reader I'd expect some kind of explanation for why this is only happening to Twilight.
Lastly, I'd like to say that you continue to do very well in terms of composition, color, and pony anatomy. The elements within the panel balance nicely, the characters all have very natural, show-quality poses, and your color choices tie everything together. Of particular merit is how well you drew a 3/4 shot of Twilight's head from behind; I know from experience that it's very difficult to get this to look right, and you made it work well here.
Best of luck with college, and I eagerly await your next panel!
-Chessie