Arthur is hurt by mostly boulder level attacks and at times lesser like that grenade launcher.
So Thanos can probably take him down with one good hit.
However Arthur and Orm were sending each other flying. Now they weren't flying a very massive distance but it was still underwater so it took effort.
Basically I think Arthur can hold his own and possibly one shot with the trident if he hits the right spot but Thanos is much stronger and one decent punch would do the trick.