If you're battling Stitches Jr. from this week's beta build, he's bugged. He shouldn't have 2 other pets on his team (both protected by his Iron Thread passive). So if you're able to defeat just Stitches, you're good. Don't worry about actually finishing the full fight since the last two pets won't be there once the encounter is fixed.
Feel free to browse through Perks N Peeves, my vanity pet collecting blog!