Why was it so easy for thor to defeat thanos It seems every other hero was just falling apart in front of him but thor could kill him in a couple of seconds.
Because the writer's pen is the mightiest weapon. Why would Thanos need ALL the infinity gems for such a small act as deleting half of all life? I mean, "reality" gem alone should be able to do pretty much anything by itself. Like I dunno, rewrite reality to only need one gem would be a good start. But that would've made for a very short, boring story. Also way less money.
But honestly, who else could? Thor is a god, every one else is a people. Also Hulk pussied out.