"Shiryu stabbed Garp from behind, Squardo style"
Turned out it is not stab from behind. Not sneak attack. Not even taking advantage of Garp's old age or injury Akainu style when he took advantage when Whitebeard was coughing up blood. Not whatever cheap trick.
Shiryu simply uses his brain, to eliminate the weakest target first.
Shiryu > Katakuri who stabbed his own abdomen for fAiR fIgHt.
Shiryu > King who randomly turned off his Lunarian durability.
Shiryu > Aokiji whose attacks hit Garp multiple times yet Garp still endured it. Shiryu stabbed him once and Garp became really weakened.
Pretty clear among all the attacks Garp ate, the one gamechanger is Shiryu's stab.