Will level 73 Rayquaza one-shot a level 70 Groudon with waterfall?

Question

It has 242 Special Attack. I don't know its IVs, but I'm assuming they're as high as possible because I don't want to kill Groudon. My Rayquaza has a Serious nature. It used Ancient Power once. I'm assuming Groudon has no IVs, nor EVs, as I'm being careful. It has about 90% of its health left, so take that into consideration. I want to catch it, so one-shotting it isn't an option. I do have a Master Ball, but I'm saving it for either Kyogre or one of the Regis.

Comments

Is the 242 special attack its evs or its actual stat?

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You can just use waterfall and soft-reset if Groudon faints. This works for every legendary, so I think it's better to save your master ball for a shiny Pokemon that might know roar or self-destruct.
For really answering the question, waterfall is physical, so what does Rayquaza's summary say is its attack stat? And when it used ancient power, did its stats rise? Does Rayquaza have any items or status effects?

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@Chickenboy it is impossible for a lv 73 Rayquaza to have a final SpA stat of 252, so it's safe to assume s/he's talking about EVs.
@sumwun all water attacks are special in Gen 3, which I'm assuming this is.

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Note: the rayquaza actually did have 242 special attack. I'm absolutely not talking about EV's. It hasn't EV trained. The game also frequently auto-saves, so pre-saving was not an option. The battle was taking forever so I used a master ball anyways. This post is outdated, but thanks for all the information.

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I just rechecked and I have no idea why I said it's impossible. I must have gotten a digit confused or something, because 252 is a perfectly reasonable SpA for a lv 73 Rayquaza.

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Why is everyone referring to the Sp. Atk stat as 252. It was 242, as stated in the post.

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No it was only KRLW. The chicken person and I both read it right.

Answer 1

Groudon with 0 SpD IVs, -SpD nature:
252 SpA Rayquaza Waterfall vs. 0 HP / 0- SpD Groudon: 277-326 (81.2 - 95.6%) -- guaranteed 2HKO

Groudon with 0 SpD IVs, neutral nature:
252 SpA Rayquaza Waterfall vs. 0 HP / 0 SpD Groudon: 248-292 (72.7 - 85.6%) -- guaranteed 2HKO

Source

I'm assuming you're in Gen 3, since you gave your SpA EVs. If Rayquaza didn't have Air Lock, Drought would weaken Waterfall enough to safely use it. But since Air Lock negates that, you do run the risk of getting either a high damage roll against the absolute lowest SpD Groudon can have (unlikely) or critting (much more likely, at 6.25%), both of which will be enough to KO after the Ancient Power you already used. Both are pretty unlikely, but if for some reason you cannot afford to lose this Groudon (like if you forgot to tell us that it's a shiny or something), then be aware that there is a risk.
Assuming, of course, that you kept you haven't continued the battle in the 10 hours it took for you to get an answer, which I suppose is rather unlikely.

Comments

Your calculation doesn't seem to consider levels. A level 73 Groudon should have 220 HP, so all of these should be OHKOs.

Answer 2

0+ SpA Rayquaza Waterfall vs. 0 HP / 0- SpD Groudon: 193-228 (87.7 - 103.6%) -- 25% chance to OHKO

source
Rayquaza was level 73, and Groudon was level 70. Groudon had minimum IVs and a -special defense nature. Rayquaza had 26 special attack IVs and 0 EVs to have exactly 242 special attack, and then I gave it a +special attack nature to simulate the badge boost.