Based on the trailer for Avengers: Infinity War, Thanos will prove to be a threat that Earth's Mightiest Heroes won't be able to take down in head-to-head combat. The Avengers will surely have to utilize unexpected methods to topple the Mad Titan, and Guardians of the Galaxy Vol. 2's Mantis might just be the one to do it, as posited by one Reddit user.
In her film debut, Mantis (Pom Klementieff) demonstrated that she had powerful empathic abilities, instantly being able to feel the emotions being felt by anyone she made physical contact with. Another way she could harness her abilities was by putting the Celestial Ego (Kurt Russell) to sleep, as she could affect his consciousness in ways that even he couldn't control.
When compared to other characters who will appear in the two-part Avengers epic, Mantis is one of the least likely to succeed in any sort of physical confrontation, barely showing any physical prowess in Guardians of the Galaxy Vol. 2. This could potentially lead to a situation in which many of the Avengers attempt to merely slow down Thanos' (Josh Brolin) reign of terror long enough for Mantis to get close to him to put him into a sleep.
Given Thanos' brief appearances in the Marvel Cinematic Universe, very little is known about the character, his abilities
Comic book fans who are familiar with Thanos' backstory know that one of his main motivations has been to court Mistress Death, which was the impetus to create the Infinity Gauntlet in the first place.
In Marvel's The Avengers, Thanos is told that to attempt to invade Earth would be an attempt to "court death itself," which many audiences may have assumed only had one interpretation.
It's possible that Thanos wants to invade Earth to cause as much destruction as possible to impress Mistress Death, which would effectively be "courting death" and possibly win her love and admiration.
Audiences will find out what role Mantis plays in Thanos' downfall when Avengers: Infinity War hits theaters on May 4, 2018.