Ariadne was ON FIRE NOW! Sure, she'd only cast ONE spell, but she'd actually gotten started, and in her very first duel, that was GOOD. Kat had wished her good luck, though, and had Ariadne had time, she would have done the same. At least they both knew they were starting out on a positive note toward one another.
Her opponent started off with a Shield Charm to protect herself from Ariadne's casting. The spell was coming right back at her, too, along with a Rictumsempra. MERLIN. Kat was good! Ariadne was left with no other choice but to cast the Shield Charm herself.
"Protego!" she called out, making a slashing motion with her wand.
A shield formed around her, and since Ariadne hadn't thought ahead on what she wanted to cast next, the spell hadn't been sent right back to Kat. Thinking fast, Ariadne came up with a spell that would be HILARIOUS if it hit Kat.
"Tarantallegra!" she called out. She whipped her wand in a crooked, upward hill motion.
Take that, Kat!