In one corner, you have the most practical/no BS Saiyan in Future Trunks, defender of his timeline's Earth, and hero to the people. Since his DBZ days, he's grown by leaps and bounds. He was mentored by Shin, the Supreme Kai, to where he mastered Super Saiyan 2 and defeated Dabura and Babidi of his timeline before they could revive Buu. He would continue to train, not for personal pride or arrogance, but to protect his Earth. During his arc in Super, Trunks hit a further level of Super Saiyan, seemingly a hybrid of Blue and Super Saiyan 2, which allowed him to at least match Goku Black in one on one, and even surge to where he could stop Fused Zamasu and deliver a brutal blow. But after this, Trunks would be dealt the severe blow of being 1 of only 2 survivors of his universe. The rest, from his mother down to little children, and everyone else, was either killed or erased.
In the other corner, we have the self-appointed dictator of the Injustice Universe, Injustice Superman. After being gassed by a special Joker toxin when Lois Lane was kidnapped, he accidentally killed her, which in turn, ignited a nuclear bomb that wiped out Metropolis. From that point forward, this Clark began going more and more Darth Vader (though without the self-loathing, it seems). It began with killing Joker, but then kept escalating to where he not only murdered Billy Batson in cold blood, but nearly wiping out Gotham and the rebuilt Metropolis. During this time period, Clark himself, or those closely associated with him, would also become personally responsible for killing people they once called friends. Showing that even old attachments would come to mean little to the Dictator of Steel.
In this battle, there will be a few scenarios;
1. Blood lust. No questions, little chit-chat, just the two getting down to business.
2. Both do a bit of talking before and during the fight.
3. Trunks becomes a sort of third party. Agreeing that Joker should have died, but that Clark went too far in what he was doing (not being on either side, but still seeing Clark as the bigger threat).
4. They sit and talk.
Arena- Ruins of a city. Be it Metropolis or West City, probably makes little difference at this point.
2. Trunks learns