The only time I'd ever worried about it being specially defensive is if I see it using Curse. Otherwise, I'll safely bring in my Specs Shandera on the revenge or something and OHKO with HP Fighting. If it's taken some prior damage, I really don't have to care which variant it is; Shandera will trap and kill it. Just the fact that it can feasibly trap and kill its own counter puts it on an entirely different level.
Then you must be facing really bad players, sir. There's a lot of stuff that can't be ohko'd in the current 'metagame', defensive or not. Actually, there isn't a single dragon that dies in one hit, while they can all set up or just ko back. A remotely decent player will always use a curser with a lot of special defense, and if it's already cursed even once, will kill you back (snorlax, swampert, t-tar, even quagsire for god's sake. Abagoora will lol at you with stab, se, life orb +1 aqua jet).
Edit: T-Tar doesnt die to HP fighting even after stealth rock, and nails back with Stone Edge, EQ, Crunch, or Pursuit.
Honestly, I'm extremely surprised seeing people still have trouble with Shandera.
Edit 2: I take it you haven't been playing too much, since weather teams are extremely common (namely sand and rain, to a bit lesser extent). All 4 weathers stop cold Shandera. In the sand, T-Tar, Garchomp, Doryuuzu, Randorosu, Blissey, you name it- none of them minds Scarf Shandera too much. Rain teams will just oustpeed and OHKO, not much to argue here. Sun is a mixed bag: Boosted STAB hurt more than Kingdra's Waterfall, but a lot of stuff will outrun, damage, status, or ohko (Blaziken's Stone Edge, Venusaur's EQ or sleep powder, Urugamoth walls you to no end, etc). Finally, hail stall. Raw special power won't get past Walrein or Regice. Regice will eventually outstall, and Shandera just cannot touch Walrein. Mamoswine can carry a scarf, resist with thick fat and ohko back, or just dodge the fire move with snow cloak.
