All you need to do is deal the final blow with a monster that has 4000 or more ATK on Ranked/Event duel.
I did this with U.A. Dreadnought Dunker in my U.A. deck, his ATK was on +500 from the field spell, and +1000 from U.A. Powered Jersey spell.
Obviously there are many different ways you can do this trophy, as there are lots of monsters that can be summoned with +4000 ATK or they can get their ATK boosted.
My opponent was on 3800, I attacked him for 4000 to win the game and the trophy unlocked!