The stage begins with 1 HaterBaa 2.0 which spawns at 100f.
When the enemy base is at less than 99% HP, groups of HaterBaa 2.0 spawns at 40-80f which stops after 15 HaterBaa 2.0s. A debuffed Baa Baa appears as the boss. Baa Baa spawns at 120-240f which stops after 5 Baa Baa spawn.
As you may have inferred(or maybe not) HaterBaa is a Metal enemy. It has a pathetic 10 HP, but there's a reason this is an Expert Stage. It has a whopping 13,500 damage, so bring cats that have a lot of HP. The stage spawns 15 of them after you hit the base, so bring critical hitters like Catasaurus or Hayabusa.