Alright so, after more than a bit of frustration with the Upgrade Bot challenge tonight, I decided to go back to the normal Endless Mode to try and lift my spirits a bit. I managed to make it to the "debut" level for Spider-Tron 6000 and cleared out the rest of the bots as usual after cutting down one of Spider-Tron's legs. I could still hear the explosions behind me, but I figured they were still far enough away and Spider-Tron was just shooting aimlessly as usual with one leg damaged. But when I went to go finish him off, he wasn't there. I could still hear the bombs and even see a few bomb particles, but no sign of Spider-Tron. After a bit of searching and following the sounds of the bombs, I decided to jump into the saw blades and see if maybe coming back as a clone might help.
The good news is that it did. The bad news is that Spider-Tron apparently decided to debut a secret move out of spite for cutting off it's leg and shooting it in the eye, because it did this right as it reappeared.