- With the buff to Knock-Off, being it super-effective against Delphox, it's probable it'll be used against her. It's worth to consider Magician as the ability on any set because of this.
- Pokémon with the Frisk ability are good partners if choosing Magician, in order to scout the items to steal.
- Azumarill is a great partner due to coverage and physical bulkness.
Excuse me, but why are these three mentioned in checks and counters, especially the Knock-Off comment? I'd say Magician as a whole should be mentioned in OO, as sets with this ability are rather gimmicky to begin with, while Azumarill as a partner should be mentioned in the respective sets.
- Weakness to common priority moves like Sucker Punch, Aqua Jet and Shadow Sneak
This sounds like a con rather than a check/counter. Mention a few examples instead, more specifically Azumarill and Aegislash, both of which OHKO-2HKO Delphox depending on their sets.
Calculations:
252+ Atk Aegislash-Blade Shadow Sneak vs. 0 HP / 4 Def Delphox: 210-248 (72.1 - 85.2%) -- 68.8% chance to OHKO after Stealth Rock
4 Atk Aegislash-Blade Shadow Sneak vs. 0 HP / 4 Def Delphox: 162-192 (55.6 - 65.9%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 4 Def Delphox: 206-246 (70.7 - 84.5%) -- 62.5% chance to OHKO after Stealth Rock
252+ Atk Choice Band Huge Power Azumarill Aqua Jet vs. 0 HP / 4 Def Delphox: 312-368 (107.2 - 126.4%) -- guaranteed OHKO
Also mention Talonflame as a very good check, only fearing Psyshock/Psychic and Switcheroo.
Calculations:
252+ Atk Choice Band Talonflame Brave Bird vs. 0 HP / 4 Def Delphox: 306-361 (105.1 - 124%) -- guaranteed OHKO
252+ Atk Life Orb Talonflame Brave Bird vs. 0 HP / 4 Def Delphox: 265-313 (91 - 107.5%) -- guaranteed OHKO after Stealth Rock
252+ Atk Talonflame Brave Bird vs. 0 HP / 4 Def Delphox: 204-241 (70.1 - 82.8%) -- 62.5% chance to OHKO after Stealth Rock