There are actually quite a few checks AV Serp handles better
Starmie's Ice Beam, Raikou's HP Ice, Azelf's Flamethrower (remember these 3 outspeed Serperior with HP Fire due to max 30 Speed IV), Thundurus' HP Ice/Flying, Keldeo's HP Flying (for safer switch-in), Slowbro's Ice Beam/Fire Blast...