To confirm: If Shell Side Arm uses Attack in cases where Slowbro-G attacks a Pokemon with higher Special Defense, does that mean we shouldn't use an attack-decreasing natures like Modest? In that case, what nature and EV spread would be ideal? We can't put EVs into Attack just for situational reasons, right? I was under the impression that SSA always uses Special Attack (like Psyshock), but that doesn't seem to be the case.
It’s a little bit complex to understand.
So the way SSA works is this;
((((2 * the user's level / 5 + 2) * 90 * X) / Y) / 50)
reduced:
((3,780 * X) / Y)/50
X being the respective offensive stat of the user after stages and Y being the respective defensive stat of the target after stages.
In most scenarios, SSA will be doing absolutely the most damage it can.
To illustrate, here is a +2 Sludge Bomb vs a Poison Jab (90 BP to be accurate)
0- Atk Slowbro-Galar Poison Jab (90 BP) vs. 248 HP / 8 Def Eviolite Chansey: 285-336 (40.5 - 47.7%) -- guaranteed 3HKO
+2 252+ SpA Slowbro-Galar Sludge Bomb vs. 248 HP / 252+ SpD Eviolite Chansey: 126-148 (17.9 - 21%) -- possible 5HKO
In this scenario, despite being +2 and invested, 90 BP Poison Jab will be doing more, so Shell Side Arm will become a physical attack.
How ever, this actually can be tricked as Shell Side Arm does not factor in things such as Burns, Fluffy, Tough Claws (as SSA will be contact based when physical), ect.
Basically, if a Burned SSA will do less as a Physical move than a Special one, and a Healthy SSA will do more damage than that Special SSA, Shell Side Arm will still become Physical.
0- Atk Slowbro-Galar Poison Jab (90 BP) vs. 248 HP / 252+ Def Eviolite Chansey: 117-138 (16.6 - 19.6%) -- possible 6HKO
252+ SpA Slowbro-Galar Sludge Bomb vs. 248 HP / 8 SpD Eviolite Chansey: 85-102 (12 - 14.5%) -- possible 7HKO
0- Atk burned Slowbro-Galar Poison Jab (90 BP) vs. 248 HP / 252+ Def Eviolite Chansey: 58-69 (8.2 - 9.8%) -- possibly the worst move ever
In this case, Sludge Bomb should be doing more damage than the 90 BP Poison Jab, but SSA doesn’t realize Slowbro is burned and still thinks 90 BP Poison Jab will be doing more, so it will become Physical.
So in better conditions, SSA will be doing the most damage it can do, but with something like Burns screwing it over and damage reducing abilities screwing it over too, you might as well still use Sludge Bomb imo, as you’re not a Special Attacker worried about Burns (and in lower tiers, Fluffy too).
You do sacrifice the extra damage on Special Walls, but it’s so little without investment and you’ll be screwed over more my them if you get burned.