I would take the alprazolam, then a large but comfortable dose of DXM, and then administer a lethal dose of fentanyl, GHB, ketamine or alcohol intravenously or rectally. In case of the latter, I would also have to make sure that the medication with DXM doesn't contain sugar, because that would be likely to cause diarrhea.
I don't have a lot of experience with pharmaceutical drugs, but DXM combined with enough alcohol for example should be 100% lethal.
My only concern would be the body trying to get rid of the substances, so I'd consider taking a normally recommended dose of antiemetics, and I'd also use other ways to prevent emptying the liquid in case of boofing.
Sedatives are complementary, although I wouldn't use diphenhydramine because of its unpleasant effects of OD.
I don't think it's realistic to expect a fatality rate from your method anywhere near 100%.
But don't take my word for any of this.
Short answer: don't attempt this method the way you described it.