A weakened Ace can defend against a Admiral. Law is getting help from Luffy and can barely defend themselves against a Schichibukai.
are you talking about when he blocked an attack from Aokiji?
You do realize that weakened or not there isn't any evidence that suggests that his DF abilities get weaker along with him. his attack would of been just as effective weather he was at 100% or dying...Also Fire>Ice so it makes sense that he blocked it but look at what happened when he went up against Akainu, he basically got one shotted Lol
If you want to use defending against an admiral then Law defended against Fujitora in greenbit all while trying to conserve his power so that he could fight against Doflamingo U_U
Hell even crocodile, who was in impel down longer than ace and probably in an even weaker state after fighting his way out still managed to slice Akainu in half and would of probably killed Akainu if he had CoA