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I started using Tyranitar a couple of days ago, and I've been on this situation a couple of times already: switch into Thundurus as it uses either a weak Thunderbolt, Taunt or Nasty Plot, then avoid a Focus Blast and OHKO it with Stone Edge (or pursuit that time it was at 25%).
AV TTar in the sand avoids an OHKO from Thundy's Focus Miss all the time, but that one who used Nasty Plot was going to sweep my whole team.