Does a prior strike defense exist?

Attached to defendant's petition for writ of habeas corpus is a signed declaration by defendant's trial attorney. In this declaration, defendant's trial attorney states that he "did not believe there was a defense to the alleged prior strike conviction," and that he "did not advise [defendant] that he had a defense to the prior strike conviction." Furthermore, defendant's trial counsel declared that defendant "expressed a desire to take his case to trial," but that he informed defendant that the maximum possible sentence for his charges was 20 years in prison, and it would have increased to 25 years since the district attorney's office "wanted to amend the information to add a prior serious felony conviction." Defendant's counsel further stated that with an amendment to the information to add a prior serious conviction, the minimum possible sentence would be nine years if defendant was convicted and the allegations proven, unless the trial court were to strike a prior strike pursuant to People v. Superior Court (Romero) (1996) 13 Cal.4th 497, which defendant's trial counsel "believed to be unlikely."