At 3 seconds: Baa Baa begins spawning every 4 - 20 seconds.
At 20 seconds: Owlbrow begins spawning every 10 - 20 seconds.
At 30 seconds: Gory Black will begin spawning every 20 - 26.66 seconds.
When the base reaches 90%, 80%, and 70%, a single Owlbrow will spawn at each threshold.
There is no boss.
This is an easy fight with some decent meatshielding and attacker stacking. Owlbrow's range is only 340 and it attacks fairly slowly, so it gets outranged by plenty of cats and is relatively easy to stall and kill with its small HP pool. Gross Cat, Lizard Cat, Valkyrie Cat, and Bahamut Cat are all non-gacha cats that can stay out of reach if the Owlbrows are properly stalled.
Strictly speaking, the occasionally spawning Gory Black is probably more dangerous to an unprepared player than all of the Owlbrows that show up, so toss in a helpful anti-black cat or two to round out your advantage if you feel the need.
Bring two or three meatshields and the above mentioned cats and you're golden.