252+ Atk Landorus-T Earthquake vs. 252 HP / 4 Def Regigigas: 135-159 (31.8 - 37.5%)
252 SpA Mega Charizard Y Overheat vs. 252 HP / 0 SpD Regigigas in Sun: 340-402 (80.1 - 94.8%)
double ko, unless ur sash pyroar in which case lando just has to eq again the next turn or something
why are you talking about
sacred sword rak, that doesnt fucking exist, they all use close combat, and you say "it would have to run max attack" like thats some special chore that not all terraks already do:
252 Atk Life Orb Terrakion Close Combat vs. 252 HP / 4 Def Regigigas: 468-554 (110.3 - 130.6%) -- guaranteed OHKO
i dont know how you can describe intimidate as a "minor annoyance" when at -1 regigigas can't even 4HKO standard Rotom-W 100% of the time after sitrus:
-1 252+ Atk Regigigas Dizzy Punch vs. 252 HP / 116 Def Rotom-W: 84-99 (27.6 - 32.5%)
and wisp is obviously only worse
Weakness policy is absolutely needed because otherwise regigigas deals 0 damage to anything (heres another calc: 252+ Atk Regigigas Thunder Punch vs. 24 HP / 0 Def Keldeo: 230-272 (69.9 - 82.6%) -- guaranteed 2HKO)
i could keep going but i'm not going to because i'm 99% sure your post is just theorymon and lying about personal experience considering you talk about Sacred Sword Terrakion and recommend Dizzy Punch Regigigas instead of doing some basic using your brain and choosing Return, and you say STAB is replaceable when it's totally not (and EQ is pathetic: 252+ Atk Regigigas Earthquake vs. 0 HP / 0 Def Mega Diancie: 192-228 (79.6 - 94.6%) -- guaranteed 2HKO 252+ Atk Regigigas Earthquake vs. 252 HP / 0 Def Aegislash-Shield: 148-176 (45.6 - 54.3%) -- 52.3% chance to 2HKO) and also well this:
http://puu.sh/hH7xs/a952e6af19.png which shows 5 battles on the smogon doubles ladder, not 37. The ladder is bad but I find it incredibly hard to believe that you went 37 battles without running into a single viable team, if you can give screenshot proof that you actually did then i'll concede this point but regigigas still sucks.
edit: spacebass just reminded me that i forgot taunt also fucks this over a million ways