Both Garp & Shanks used Hardening on their Fist/Sword
With Garp's Fist being Portrayed as Strong as a Supreme Grade Blade (That's what Chinjao's Fight indirectly told us)
Then They applied CoC Coating which is mostly what made their Attacks so Powerful
Then They applied Internal Destruction CoA, which in Shanks Case made Kid's Weapon Explode while in Garp's Case created Bigger AoE
(Similar to Sabo's Dragon Breath in Dressrosa Colosseum)
And finally, they both Applied Barrier CoA
Shanks had to use it to Protect himself from Explosion of Kid's Weapon
While Garp used it for Range
So it's Sword/Fist + CoC Coating + Hardening + CoA Barrier + CoA Internal Destruction
And these Two Attacks were shown Back To Back
And Shanks Attack is same as Roger's who was Garp's Rival
In other words, Oda is telling us Divine Departure = Galaxy Impact