how to achive comeback victory?

You need to be vs'ing a female character (or a male if you swing that way), take them from behind and cum on their back as you finish. hence comeback victory.

Elements Unite! allows you to achieve Comeback Victories easily

achive

Comeback victory is when your lifepoints are lower than your opponent before you do your final attack to finish the opponent of.

I honestly think it's a buggy one, I've gotten it in some weird circumstances. Once when I passed on the play without dropping a monster against Insect Multiplication AI Deck and they filled the board with the ant tokens. They didn't deal any damage to me and I proceeded to steamroll but still came out with Come Back

You do comeback victory, duh

Comeback Victory happens if your LP were lower than your opponent's at the START of turn in which you won, and remained lower through the whole turn.

You need to win the duel with less than 1500 remaining LP.

Comeback Victory is achieved, when at the start of your turn your hp are less than the ones of your opponent. The best way to do this easily is level 10 Yami Yugi as he use +hp spells so he will have higher HP than you resulting in an Comeback victory. Use a tribute-beater deck, but don't attack(and lower his HP) until you can do enough damage to kill him in 1 turn. Another way to 'easy' is the God Winged Dragon of Ra as his effect let you use your hp to buff his ATK/DEF. After you used this effect you end your turn and in the next one, you kill your opponent completely.