roger was talking abt how he should be capable of hurting xebec cause he was using haki and haki doesnt yield to devil fruits .
so u can just negate the df user's defense with ur ability but in order to do that -
u need superior haki to get past his haki defense first .
luffy needed internal destruction to get around kaido's tough scales but if someone has way stronger haki than kaido then he can simply hurt him with regular attacks ( the same might go for logias )
prob the same goes for imu too , in order to fully negate his immortality ,
u need to completely penetrate his coc defense , otherwise his coc defense will cushion ur haki attack and u would be unable to negate his immortality .
imu's base set is from a df ability too
xebec was able to partially resist his domi-reverse
tho he lost his state of mind when imu talked abt herald
so in conclusion , u need 2 things in a single attack against devil fruit users -
1. penetrate their haki defense ( pretty tough if the df user is a supreme king )
2. hit them with haki strong enough to negate their df ability
it can either be regular attack or a momentarily haki burst like roger and garp had against Dr xebec .
xebec was able to hurt garling cuz his haki is way stronger than garling's haki defense but imu's haki defense is on another level so he was unable to get past imu's immortality
u just gotta hope oda will be consistent with this power system , he will not be
we can joke how the ultimate solution to a battle would be " hurr durr stronger haki "
but thats literally how its been going for the past 2 events now
egghead's saviour was a simple haki blast from joyboy and god valley's saviour was a huge mass of strong haki attack
and it looks like elbaf might be concluded with haki too