LOL, all Thanos had to do would be to find some Kryptonite - no battle at all. If Thanos is good at anything it's finding hidden secrets of the universe, so tracking down that which is littered all over Smallville would be no problem.
Yeah, and considering Thanos WITHOUT the IG is on par with freakin' BROLY... Supes might go toe-to-toe with him, but lest he finds a Blue Star and immerses himself onto its core for at least 10 minutes, he's toast.
See, that's a common misconception that gets wanked to the moon and back. We saw something happen to the South Galaxy, but we're not sure how it happened or how long it took for that to happen. For all we know, Broly and Paragus planet hoped for 18 to 30 years and slowly destroyed the South Galaxy over time. This would be completely in line with what we know high end chi users are capable of. Going by quantifiable feats, Broly is at best a planet buster like practically all the DB cast.