This is going to be an interesting battle.
T-1000 can absorb D.Va's infinite bullets, and will even regenerate his health. Yet, T-1000's regeneration abilities can be stun-locked out of use with high firepower, something which D.Va can do with her mech's fusion cannons. However, her fusion cannons become more powerful the closer she gets to her opponent, and she doesn't want to get into a close-quarters combat with T-1000, as he'll instantly wreck her suit... but, D.Va has boosters, and a Defense Matrix system, to aid in her escape, and create some distance between the two. Considering both of those have relatively low cool-downs, by the time T-1000 gets close, D.Va will be able to use these abilities again.
This will be a cat-and-mouse game for T-1000 and D.Va, but considering this is taking place inside a high school, with corridors and many twists, D.Va can use her boosters to her advantage, allowing even further separation. For those that say D.Va cannot use her robotics to outmanoeuvre the T-1000, may I remind people that the T-1000 was produced in 2029. In the Overwatch lore, the current game takes place sixty years into the future, meaning it'd be 2076. D.Va, being 19 years of age, would have been brought into the MEKA program very recent in the OW lore, at earliest 2074... that's almost 50 years of difference in technology... not to mention the fact that the MEKA was created to fight in the Omnic Crisis, a battle involving many robotic enemies, which I'd argue to say aren't too similar to the T-1000. So, I can only assume that fifty years of difference in technology, considering how far we've come in the last 20 or 30, will cause T-1000 to be outclassed by the MEKA.
The big finish comes from D.Va activating her self destruct sequence, as ballistic firepower is one of T-1000's weaknesses. If a grenade blast can render a T-1000 temporarily useless, then the power emitted from the self destruct, one which can level massive omnic robots, should be able to defeat T-1000 with ease. As long as she keeps using her senses, keep distance, and pot-shotting T-1000 for her ultimate charge, all D.Va needs to do is lure the T-1000 into an open area, and launch that self destruct... since the self destruct can be countered by hiding behind a strong wall.
With Hana Song being an intense gamer, being one of the best PvPers in the world, she'd have amazing sense of her surroundings, and will be easily able to outplay an AI; an enemy on the PvE. I know there are some tough PvE games, and bosses, but they do not stack up to the difficulty that is Pro Gaming... and for something as outdated as the T-1000 (compared to the MEKA, as explained), D.Va should be able to easily trick D.Va into an area where the self-destruct will melt the T-1000 into oblivion.
D.Va uses her superior technology and wits (together, and separately) to outplay the old T-1000 model, blowing it up with the self destruct.
