Note: The Ultimate Edition (PS4 version) features 30 fighters but I can confirm only the 24 originals are needed for this trophy. As was the case with the PS3 & 360 versions.
The quickest way to unlock this trophy is to play on 'very easy'. Each 'classic battle' will take roughly 5-10 minutes each so this will likely take roughly 3-4 hours in total.
Fighters not required: Martian Manhunter, Zatanna, Batgirl, Lobo, Zod & Scorpion.
Have fun!