Focus Blast does 98.5% minimum to T-Tar, a very likely OHKO and a definite OHKO with SR up. Running Rash and taking the special defense drop involved, Focus Punch does 85.4-101% (this is assuming T-Tar is curse EVd, the worst case scenario, 252HP/40Def/216S.Def).
On Blissey, Focus Punch does...