I chose not to use Heavy Slam because I wanted a Special Attacker Celesteela, and I can't try for another nature because I've already done the UB story. I also mistyped my EV spread; there is a Max HP Spread in there somewhere.
And I don't like Alolan Marowak, so I'm not using it. Simple as that.
Most Fairy Pokemon are specially defensive or naturally have better Special Defense than Defense, meaning they can easily take a Flash Cannon. OHKOing Fairies with Heavy Slam outright (or bringing them close to it) is better than Flash Cannon leaving them alive long enough for them to switch out:
152 SpA Celesteela Flash Cannon vs. 252 HP / 84+ SpD Clefable: 192-228 (48.7 - 57.8%) -- 50.8% chance to 2HKO after Leftovers recovery
0 Atk Celesteela Heavy Slam (120 BP) vs. 252 HP / 172 Def Clefable: 272-324 (69 - 82.2%) -- guaranteed 2HKO after Leftovers recovery
152 SpA Celesteela Flash Cannon vs. 0 HP / 0 SpD Altaria-Mega: 204-240 (70.1 - 82.4%) -- guaranteed 2HKO after Stealth Rock
0 Atk Celesteela Heavy Slam (120 BP) vs. 0 HP / 4 Def Altaria-Mega: 240-284 (82.4 - 97.5%) -- 62.5% chance to OHKO after Stealth Rock
152 SpA Celesteela Flash Cannon vs. 0 HP / 0 SpD Tapu Lele: 186-222 (66.1 - 79%) -- guaranteed 2HKO after Stealth Rock
0 Atk Celesteela Heavy Slam (120 BP) vs. 0 HP / 0 Def Tapu Lele: 330-390 (117.4 - 138.7%) -- guaranteed OHKO
And I hate to be blunt here, but there are times when the Pokemon you don't like are better choices in standard play than personal preferences. Give them a try before you knock them. Do some tests with the suggested changes, then report back. Another reason why Marowak is better over Salazzle is because it pairs well with Celesteela by covering its Electric weakness with Lightning Rod.