I've been on this same quest for like two days now. I am currently doing the Interrogate the Interrogators. My pet is level 16 but he keeps getting beat by those three Nightfallers.

what is the lvl of the nightfallers? Sorry, I have a bad memory

One way to beat bosses is to grind your pets to a higher level, maybe 5 levels or more for easier battling.

Another way is to get better equip. Try to sell all your green equip, and get at least blue equipment for your party pets. You will find that this does huge improvements to battling

The third way is to review your party structure. I think that at lvl 16, there is only 1 party slot available. But if not, another way is to unlock the second slot and check out the party guides to see what you need to do to build an effective party.

Hope this helps

Thanks. I will try those suggestions.

Edit: The bosses are all level 15.