Transformers: The Movie opens with a full assault by the Decepticons on Autobot City, the climax of which is the final duel between Megatron and Optimus Prime, one-on-one, and we all knew that the larger battle depended on the outcome of this fight more than anything else. Although Megatron fought with a hunk of shrapnel, a laser sword, and his arm cannon, Optimus Prime ultimately proves to be his better.
Seeing a discarded pistol behind some debris, Megatron begs for mercy before Optimus Prime can finish him off once and for all, trying to buy precious seconds to grab the pistol. Hot Rod, a new character to the series, sees what is happening and tries to stop him, but winds up getting in the way and being used as a shield while Megatron blasts Optimus Prime at point blank range over and over again, until he finally collapses.
"I would've waited an eternity for this.
It's over, Prime!"
Thinking Optimus Prime defeated, Megatron discards Hot Rod and levels the pistol at the fallen Autobot leader, but he is able to summon one final surge of strength to knock Megatron off a ledge with a last, desperate two-fisted blow. With the already badly injured Megatron now damaged beyond his ability to fight, Starscream takes command of the Decepticons and calls for a retreat. Although Autobot City was destroyed and their leader's wounds eventually proved fatal, the Autobots won the day.
Among those who grew up in the 1980s, this line is synonymous with the classic supervillain hubris that always seems to give the hero one last chance to snatch victory from the jaws of defeat.
Source - Transformers: The Movie DVD Chapter 7 (You Got the Power)