How do you know Kizaru wasn't trying? His main objetive wasn't Luffy but that literally played in his favor (ran away from Luffy, gaining time, Luffy finally fell down do to being out of time), and he still used multiple techniques we had never seen before.
Regardless, both Doflamingo and Kaido took multiple hits from Luffy (and other people) before taking the hit that took them down. Kizaru only took one hit. One shot.