
Every version of Donatello has at least some experience with extra-terrestrial technology. Countless times, he’s helped the Teenage Mutant Ninja Turtles overcome a technologically superior enemy, and the Predator is no different.
This being a random encounter, I don’t expect Donnie to be carrying a suitcase full of tools and gadgetry, but he always has a few tricks up his sleeve.
Donnie would first find a way to neutralize the Predator’s cloaking technology so he could see what he was dealing with. From there, Donatello would find a way to either turn the Predator’s tech against it or eliminate it from the equation altogether.
Sensing that a hand-to-hand fight would be suicide, Donnie would likely set up some traps, ala Arnold in the first Predator (1987), to incapacitate or at least slow the creature down.
Ultimately, it comes down to whether Donatello kills the Predator or simply leaves it immobilized. Of all the Teenage Mutant Ninja Turtles, Donatello is most reluctant to kill—his weapon is a big stick, after all—so instead, he knocks the Predator out and safely retreats back to the sewers for reinforcements.