You used a regular Lopunny, not Mega oneHard Counter
Slowbro @ Leftovers / Slowbronite
EVs: 252 HP / 252 Def / 4 SpD
- Slack Off
- Thunder Wave / Calm Mind
Slowbro does not fear any of Lopunny's STAB damage whatsoever, not even if Lopunny is somehow able to setup beforehand. Lopunny doesn't do anything at all to Slowbro. Even if it carries Thunder Punch (which is extremely rare), it would take five turns in order to potentially finish Slowbro off (and it would never happen anyways, because Slowbro can keep healing itself and will finish Lopunny way before it happens and might cripple it completely in the proccess).
On the other hand, Lopunny must fear being potentially 2-3 K.Oed with ease, being burned or even being used as a setup fodder if Slowbro is carrying a Calm Mind variant as its set. If Lopunny switches out, its trainer must also watch out for a potential cripple on whichiever Pokémon it choses to respond to Slowbro.
252 Atk Lopunny Return vs. 252 HP / 252+ Def Slowbro: 79-94 (20 - 23.8%) -- guaranteed 5HKO
252 Atk Lopunny Thunder Punch vs. 252 HP / 252+ Def Slowbro: 78-94 (19.7 - 23.8%) -- possible 5HKO
+1 252 Atk Lopunny Return vs. 252 HP / 252+ Def Slowbro: 118-141 (29.9 - 35.7%) -- 35.1% chance to 3HKO
252 Atk Lopunny Return vs. 252 HP / 252+ Def Slowbro-Mega: 55-66 (13.9 - 16.7%) -- possible 6HKO