No luffy never beat saturn like gaban beat sommers. Sommers was down for much longer for example.
Roger pirates were always beating HKs like gaban did to sommers but as they had regen they eventually healed and kept coming back.
Gaban even tells luffy about it and luffy now says he knows what to do.
There is also an argument that luffy never used acoc against gorosei. At least visually.
Gaban him self said that they never face any HK that had CoC.
You can't compare sommers to the elders when they all have CoC .
We see in this flashback Gaban was doing attack and harald was able to heal easy and Gaban said it could be because of CoC.
Gaban and Shanks were even unable to slow him down really.
The argument that luffy never used acoc against elders is one i know of but it sound so stupid it crazy.
When he said he try everything .
But with the way how Oda draw haki guess we won't know.