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That's why you run Taunt.
I think you meant 252 HP/ 252 SpD Gigalith cuz 252 HP/ 0 SpD Gigalith is definitely an OHKO with Hydro Pump from Greninja even if with +Sp.def nature. Though it is a roll... (252 SpA Greninja Hydro Pump vs. 252 HP / 0+ SpD Gigalith: 176-210 (91.6 - 109.3%) -- 56.3%...
I don't know what happened but I was not able to register for the competition. For some reason my game was still registering my 3DS as participating in the Little Cup online competition a few months back, so when I put in my legends team in the Battle Box and went to register, it wouldn't accept...