Is there really a reason to put those EVs in Def over HP, though? Yes, you have the Assault Vest to boost its bulk, but it just seems like going fully on HP does more good regardless of what bulk you want to specialise in.
True enough. Honestly I'd have to do some calcs to see if Def would really benefit that set over HP, but you're more than likely right.
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Aight, I'm gonna test this with standard Granbull with Play Rough, Close Combat, and Earthquake (also Crunch, but we won't use that since it's a double-resisted move), Specs Mismagius with Shadow Ball, Energy Ball, and Dazzling Gleam (also has Power Gem, but again, that's not tested because it's non-STAB and resisted), and Assault Vest Florges with Moonblast and Energy Ball (not testing Psychic since Scrafty is immune to it, or Hidden Power since it's pretty weak anyway and wouldn't benefit much even if it was Fighting or Flying).
252 HP, 252 Att, 4 SpDef, Adamant:
252+ Atk Granbull Play Rough vs. 252 HP / 0 Def Scrafty: 540-640 (161.6 - 191.6%) -- guaranteed OHKO
252+ Atk Granbull Close Combat vs. 252 HP / 0 Def Scrafty: 240-284 (71.8 - 85%) -- guaranteed 2HKO
252+ Atk Granbull Earthquake vs. 252 HP / 0 Def Scrafty: 101-119 (30.2 - 35.6%) -- 32.2% chance to 3HKO
252+ SpA Florges Moonblast vs. 252 HP / 4 SpD Assault Vest Scrafty: 364-432 (108.9 - 129.3%) -- guaranteed OHKO
252+ SpA Florges Energy Ball vs. 252 HP / 4 SpD Assault Vest Scrafty: 58-69 (17.3 - 20.6%) -- possible 5HKO
252 SpA Choice Specs Mismagius Dazzling Gleam vs. 252 HP / 4 SpD Assault Vest Scrafty: 268-316 (80.2 - 94.6%) -- guaranteed 2HKO
252 SpA Choice Specs Mismagius Energy Ball vs. 252 HP / 4 SpD Assault Vest Scrafty: 75-89 (22.4 - 26.6%) -- 24.4% chance to 4HKO
252 SpA Choice Specs Mismagius Shadow Ball vs. 252 HP / 4 SpD Assault Vest Scrafty: 50-59 (14.9 - 17.6%) -- possible 6HKO
4 HP, 252 Att, 252 Def, Impish:
252+ Atk Granbull Play Rough vs. 4 HP / 252+ Def Scrafty: 400-472 (147 - 173.5%) -- guaranteed OHKO
252+ Atk Granbull Close Combat vs. 4 HP / 252+ Def Scrafty: 178-210 (65.4 - 77.2%) -- guaranteed 2HKO
252+ Atk Granbull Earthquake vs. 4 HP / 252+ Def Scrafty: 74-88 (27.2 - 32.3%) -- guaranteed 4HKO
252+ SpA Florges Moonblast vs. 4 HP / 0 SpD Assault Vest Scrafty: 372-436 (136.7 - 160.2%) -- guaranteed OHKO
252+ SpA Florges Energy Ball vs. 4 HP / 0 SpD Assault Vest Scrafty: 58-69 (21.3 - 25.3%) -- 0.1% chance to 4HKO
252 SpA Choice Specs Mismagius Dazzling Gleam vs. 4 HP / 0 SpD Assault Vest Scrafty: 268-316 (98.5 - 116.1%) -- 87.5% chance to OHKO
252 SpA Choice Specs Mismagius Energy Ball vs. 4 HP / 0 SpD Assault Vest Scrafty: 75-89 (27.5 - 32.7%) -- guaranteed 4HKO
252 SpA Choice Specs Mismagius Shadow Ball vs. 4 HP / 0 SpD Assault Vest Scrafty: 50-59 (18.3 - 21.6%) -- possible 5HKO
4 HP, 252 Att, 252 SpDef, Careful:
252+ Atk Granbull Play Rough vs. 4 HP / 0 Def Scrafty: 540-640 (198.5 - 235.2%) -- guaranteed OHKO
252+ Atk Granbull Close Combat vs. 4 HP / 0 Def Scrafty: 240-284 (88.2 - 104.4%) -- 31.3% chance to OHKO
252+ Atk Granbull Earthquake vs. 4 HP / 0 Def Scrafty: 101-119 (37.1 - 43.7%) -- guaranteed 3HKO
252+ SpA Florges Moonblast vs. 4 HP / 252+ SpD Assault Vest Scrafty: 268-324 (98.5 - 119.1%) -- 93.8% chance to OHKO
252+ SpA Florges Energy Ball vs. 4 HP / 252+ SpD Assault Vest Scrafty: 43-51 (15.8 - 18.7%) -- possible 6HKO
252 SpA Choice Specs Mismagius Dazzling Gleam vs. 4 HP / 252+ SpD Assault Vest Scrafty: 200-236 (73.5 - 86.7%) -- guaranteed 2HKO
252 SpA Choice Specs Mismagius Energy Ball vs. 4 HP / 252+ SpD Assault Vest Scrafty: 56-66 (20.5 - 24.2%) -- guaranteed 5HKO
252 SpA Choice Specs Mismagius Shadow Ball vs. 4 HP / 252+ SpD Assault Vest Scrafty: 37-44 (13.6 - 16.1%) -- possible 7HKO
252 HP, 4 Att, 252 Def, Impish:
252+ Atk Granbull Play Rough vs. 252 HP / 252+ Def Scrafty: 400-472 (119.7 - 141.3%) -- guaranteed OHKO
252+ Atk Granbull Close Combat vs. 252 HP / 252+ Def Scrafty: 178-210 (53.2 - 62.8%) -- guaranteed 2HKO
252+ Atk Granbull Earthquake vs. 252 HP / 252+ Def Scrafty: 74-88 (22.1 - 26.3%) -- 12.7% chance to 4HKO
252+ SpA Florges Moonblast vs. 252 HP / 0 SpD Assault Vest Scrafty: 372-436 (111.3 - 130.5%) -- guaranteed OHKO
252+ SpA Florges Energy Ball vs. 252 HP / 0 SpD Assault Vest Scrafty: 58-69 (17.3 - 20.6%) -- possible 5HKO
252 SpA Choice Specs Mismagius Dazzling Gleam vs. 252 HP / 0 SpD Assault Vest Scrafty: 268-316 (80.2 - 94.6%) -- guaranteed 2HKO
252 SpA Choice Specs Mismagius Energy Ball vs. 252 HP / 0 SpD Assault Vest Scrafty: 75-89 (22.4 - 26.6%) -- 24.4% chance to 4HKO
252 SpA Choice Specs Mismagius Shadow Ball vs. 252 HP / 0 SpD Assault Vest Scrafty: 50-59 (14.9 - 17.6%) -- possible 6HKO
252 HP, 4 Att, 252 SpDef, Careful:
252+ Atk Granbull Play Rough vs. 252 HP / 0 Def Scrafty: 540-640 (161.6 - 191.6%) -- guaranteed OHKO
252+ Atk Granbull Close Combat vs. 252 HP / 0 Def Scrafty: 240-284 (71.8 - 85%) -- guaranteed 2HKO
252+ Atk Granbull Earthquake vs. 252 HP / 0 Def Scrafty: 101-119 (30.2 - 35.6%) -- 32.2% chance to 3HKO
252+ SpA Florges Moonblast vs. 252 HP / 252+ SpD Assault Vest Scrafty: 268-324 (80.2 - 97%) -- guaranteed 2HKO
252+ SpA Florges Energy Ball vs. 252 HP / 252+ SpD Assault Vest Scrafty: 43-51 (12.8 - 15.2%) -- possible 7HKO
252 SpA Choice Specs Mismagius Dazzling Gleam vs. 252 HP / 252+ SpD Assault Vest Scrafty: 200-236 (59.8 - 70.6%) -- guaranteed 2HKO
252 SpA Choice Specs Mismagius Energy Ball vs. 252 HP / 252+ SpD Assault Vest Scrafty: 56-66 (16.7 - 19.7%) -- guaranteed 6HKO
252 SpA Choice Specs Mismagius Shadow Ball vs. 252 HP / 252+ SpD Assault Vest Scrafty: 37-44 (11 - 13.1%) -- possible 8HKO
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When it comes to a dedicated physical attacker with Play Rough, this set's likely to get OHKOed no matter what, and even with full HP and Def EVs a Fighting type physical attack is likely to 2HKO it, so putting EVs into Def is going to end up pretty pointless, as you said. As for a dedicated Special attacker, we have a similar scenario, in that the only way Scrafty is going to survive a Moonblast is with both HP and SpDef fully invested, so SpDef isn't especially worth it either. As you can note, though, neutral hits are easily shrugged off from all comers, even when there's only HP investment, only becoming a problem with Att/Def or Att/SpDef builds. Since Drain Punch heals more as it deals more damage, and it's still beneficial to deal as much damage as possible to an incoming pokemon, that means an HP/Att build is te most likely to give you more bang for your buck.