Most Marvel Comics heroes have taken their turn facing down the Mad Titan Thanos is obsessed with Death - not just causing it, but the metaphysical manifestation of the process, which has resulted in an unusual relationship with mortality.
In The Thanos Imperative #1 - from Dan Abnett and Andy Lanning - Thanos has just been resurrected by Oblivion, a cosmic entity who wants Thanos to act as the Avatar of Death on the mortal plane. Thankfully, the process wasn't perfect, and Thanos was resurrected early, lacking much of his usual power and genius. Having gone on a disastrous, rage-fueled rampage, Thanos is captured by the Guardians of the Galaxy and imprisoned on Knowhere, where he promptly breaks free and prepares to crush Peter Quill, aka Star-Lord.
Knowing that Thanos isn't yet back to full strength, Rocket threatens to gun him down with a mech-mounted Rampart Arms Phasic Cannon. Utilizing cold fusion, Rocket theorizes the weapon will be enough to kill Thanos in his weakened state, but reveals that isn't his plan. Instead, Rocket states he has the blast set to quarter power - just enough to paralyze Thanos for a short period. Once that's done, Rocket threatens, he will "seal you up in a cryogenic pod and drop you into a gravity sink. We know it's not death you're afraid of. It's life. And that's what I'm threatening you with. Endless life. Let. Him. Go."
It's a bold move by Rocket, who was at that time the team's master tactician. As the Avatar of Death, and possessing the of a being as powerful as Oblivion, there was no guarantee that Thanos would permanently succumb to even mortal injury, but that also meant there'd be no way out of the imprisonment Rocket envisioned, and Thanos would be forced to live forever. At this point in time, one of Thanos' chief concerns was simply to remain dead, with his hatred of life being compared to an allergy that hurt every moment he was conscious. Rocket's threat is as dark as they come, but also perfectly judged to turn Thanos' immortality into a weakness.
The threat works, and Thanos releases Quill and allows high-level psychics Moondragon, Mantis, and Cosmo to render him unconscious. The team would go on to use Thanos to combat the Cancerverse - a reality where Death is gone, and horrific lifeforms attempt to find new worlds to conquer - so not only did Rocket's gambit save Peter Quill, but it gave the team the one weapon they needed to save reality. Rocket has long been more of a rogue than a traditional hero, and it's clear he's dead serious about his threat - not least because the team eventually tried to ditch Thanos in the Cancerverse, effectively sentencing him to the exact fate Rocket described.
Thanos is rightly seen as one of Marvel's deadliest villains, but too frequently Marvel's heroes don't bother to try and see things from his perspective. Thanos' motives are more complex than conquest or even - at this point in comics history - cosmic balance. In talking Thanos out of killing Star-Lord and forcing him to submit to defeat and capture, Rocket Raccoon showed that his personal tactical brilliance could succeed where brute force and convoluted heroic schemes have failed. Thanos is a powerhouse of cosmic importance, and whether in Marvel's past or its future, defeating him will never be a matter of fighting harder or bringing a bigger gun, but of understanding his warped perspective just enough to foil each new attempt to bring chaos to the galaxy. Thankfully, with heroes like Rocket Raccoon on the job, Thanos is a threat that can be managed, if never fully overcome.
