By:
1) his invisibility.
=> Example, Luffy punch his enemy without touching. But the others only know that Sanji can go invisible, so people thinks that the one who defeat Luffy enemy is Sanji. That's for Zoro case too.
OMG, Sanji is beating some strong characters...
actually, Sanji is just lost. He goes to wherever whores appear.😤😉 He wants to save them 😒
2) making doping out of most food source he sees.
=> Example, Luffy - Zoro - anybody are exhausted. Then, they get completely recover after eat Sanji's food while their opponent are still exhausted.
Oh, this guy (Sanji) is the source of their recovery.
Banzai... Senzu beans from Dragon Ball... Banzai!