Dragon Ball Z has one of the best plot swerves in the whole franchise, setting up a massive conflict with the restored Frieza and his armies -- only for the arrival of Future Trunks to derail that plot and send the heroes into a far more frightening and exciting storyline. But the arrival of Trunks means that in his own history, Frieza was still defeated -- but this is never shown in the original series.
It turns out that, in Future Trunks' own timeline, the Z-Warriors defeat Frieza and King Cold without his help, according to a surprising source: a video game.
After his defeat and near-death on Namek, Frieza was found by scouts sent by his father, King Cold. Frieza was saved by cybernetic intervention, with his body grafted onto mechanical augments to ensure his survival. With these advancements increasing his already phenomenal power levels, Frieza decided to enact his revenge on the Saiyan who'd beaten him. With the knowledge that Goku and the rest of his allies call Earth home, Frieza -- alongside King Cold and an entire armada of soldiers -- make their way to the planet, with the intention of wiping out all of humanity. They may have even succeeded, as Frieza and his ship were quickly able to outpace Goku and his ship, which was also racing back to Earth so he could protect his loved ones.
The Z-Warriors assemble to try and counter Frieza and his forces, quickly moving into position around his ship and lowering their power levels to maintain the element of surprise. The group prepared for one of the most difficult fights of their lives, fully aware of just how strong Frieza must be -- and that King Cold could prove to be just as bad if not worse. However, in the core-Dragon Ball Z storyline, the Z-Warriors actually never had to fight either of them. A version of Trunks from an alternate future arrived on the scene and quickly dispatched the army, Frieza and King Cold with his own Super Saiyan form. Saving the Z-Warriors a great deal of drama, he waits alongside them for Goku to arrive. This way, Trunks is able to warn the hero of his fate from the original timeline, and help prevent it by changing the past.
But in that original timeline Trunks hailed from, there was no warrior from the future to arrive and stop Frieza. Instead, it fell to the Z-Warriors -- and was further expanded on in (of all places) the mobile-game Dragon Ball Z: Dokkan Battle. Featured as part of the "HOPE!! Fight Against Despair!" quest event, the scenario starts with the Z-Warriors stealthy approaching Frieza. Having already defeated Frieza's army, they actually get close to his ship. However, Frieza himself moves onto the battlefield and finds himself confronting an unlikely pair: Vegeta and Krillin. Taken aback seeing the same two warriors he killed on Namek, a pleasantly surprised Frieza decides to personally murder the both of them again, in hopes of seeing just how mad it makes Goku.
This is when Goku arrives, revealing his Instant Transmission to the assembled fighters. Frieza and King Cold are just as shocked as the Z-Warriors, but Goku announces there's no time for explanations. Fed up with Frieza and furious that he's given up so many chances to step away from their fight and survive, Goku confronts him and King Cold, quickly turning Super Saiyan and overwhelming them with the rest of the Z-Warriors by his side -- killing both of them. There are no casualties on the hero's side, meaning this is a pretty solid victory for Goku and his friends. The only hanging plot thread from the battle comes courtesy of Vegeta, who (without the Androids to focus on like in the core-Dragon Ball timeline) instead vows to overtake Goku in terms of power so he can finally defeat and even destroy him.
Not long after, however, Goku dies of the heart disease Trunks is initially sent to the past to prevent. The details of Frieza's defeat are fairly straightforward, with Goku in the core-Dragon Ball timeline even admitting he'd been likely to do the same until he sensed the arrival of Trunks and decided to let that mysterious power handle the threat. But it's still a long-lingering question from the franchise, finally answered.