And its been years since she did so. She tried to keep herself in shape while working as a waitress, but besides some Syr/Bell adventures, she has not been in the frontlines(meaning challenging floors to her), not counting that her status hasnt been updated and shes still at High lv 4, when she probably by now should be 5 or even halfway to 6
Ryuu is a veteran adventurer, but those last years she hasnt being very active besides specific moments unlike Ais whos been in the Dungeon as often as she possibly could, so in a sense she kinda has lost her edge even if shes the strongest rank 4 in Orario
Ais fighting humans is really rare compared to her usual monster fights, plus due to her trauma/obession with monsters shes unable to go all out against humans. Not like they are many that are a real threat to her (lv6 or higher human enemies) now so most of the time she doesnt have to try hard. The red haired tamer looked human but she WAS technically a monster so in her mind Ais treated her like oneRyuu can buy 3 minutes is already a miracle. She's just a lv 4, with a boost that make her a mid lv 5, again a top tier lv 6 like ais it's not even a battle. Between skill, Ais fight again rival familiar like Istar and Freya, and a lv 6 red hair tamer, while Ryu fight again lv 3,4 Evilus seven years ago. Another point is that Ais is the one who teach Bell how to fight again human opponent, she learnt that from Finn as the top guild PVP is usually occur. I don't know why you think Ryu has more experiences than Ais when Ais constantly fights against both monster and human.
I'm sure that isn't the ONLY reason...Finn actually wants to marry Lilli (he needs to marry a Pallum due to his beliefs)