There's also been a few mixed signals about the posting of different sets for the same pokemon. I'm going to assume if they're varied enough then there shouldn't be an issue since a counter to one set does not necessarily counter them all. I feel this set is sufficiently varied to warrant posting, but if a fourth TTar is pushing it at this stage I'll delete it. But yeah;
Tyranitar@Leftovers/Lum Berry
Adamant
252HP/104Att/72Def/80Spe
Dragon Dance
Stone Edge
Crunch
Low Kick
Bulky DD Tyranitar effectively counters that Reshiram build.
The EV spread allows Tyranitar to avoid being 2HKO'd by any combination of attacks from Reshiram, allowing it to set up at least one dragon dance. 80 Speed EVs allow a +1 Tyranitar to outspeed Reshiram, and I think you can imagine what a +1 STAB Stone Edge does to Reshiram (In case you can't: 161.9%-190.6%). Crunch has great coverage in Ubers and gets STAB, while Low Kick rounds out the set by hitting Dialga and Darkrai. Lum Berry can also be considered for the item in order to avoid Dark Void and Will-O-Wisp from Darkrai and Giratina respectively, giving you the opportunity to sneak in an extra DD if they switch in to counter.