Surprisingly, Act 0 actually has a mini-boss. Although I wouldn't recommend you actually fight it in Act 0. I recommend having at least Jekyll and Wraps from Act 1 before challenging them. That being said, here is how you find the boss:

The first thing you'll want to do is defeat the paranormal lamp. Once that's been done, I recommend going to Alice's room, and saving at the bed. Now all you have to do is go to the basement in your house. The basement is located right here



Congratulations, that's all you had to do. Things get a lot more complicated from here on though so be ready. This boss should be light work if you have everyone from Act 1. Bolt's shockwave is really good for this fight too.

The secret boss is technically in Act 1. However, you still need to beat Act 1 to actually fight it. This goes for most of the secret bosses so keep that in mind. Once you've done that, progress through Act 2 until you encounter Shelby. She'll tell you the roads are opening n stuff. Now go back to Act 1, and find Bonz. He stands outside the inn at Boo Boulevard. He will give you a key which you must take here:



It might not be very visible, but you can walk through there and find the shed which you can use the key to open. This boss can be quite tricky if you don't know what you're doing, but as long as you know your elements and have a good team, then you should be fine. What's important is once you beat the boss, check Jiang's shop. There should be a new item in stock. You'll need it for later.

Alright, Act 2 is gonna have you putting in some serious work for this boss. The boss should be accessible as soon as Poe moves out of the way and you're able to enter the caravan. Once that happens you gotta start playing some games. Honestly, you might be better off just buying the Poekens, since most of the games don't end up giving you that many. You'll need about 10k candy if you want to do it this way tho. Anyways once you get 500 Poekens, all you have to do is buy the Mirror Ring, and equip it. Then simply enter the room to the right of the prize counter, and walk into the center mirror.



This will take you to a weird warped version of the carnival. From here, just play games until you get 4 special Poekens, then buy the gem from the guy at the prize corner. Be sure to save BEFORE you do this tho, as you will not be able to once you buy it. Sounds easy enough, but the games played for the special Poekens are notably harder than the original versions of the game, so be ready!

Act 3 has my favorite secret boss by far. However, it'll take some serious time before you'll be able to fight it. Thankfully you have my perfect guide to get you there as soon as possible! This boss requires that you recruit Flynt and Jack before you can fight it. They're both pretty easy to recruit, so not much is needed to be explained there. Once you've done that, you'll also want to beat Act 3. After both those things are taken care of, head to the Gloomy Basin with Flynt in your party, and find all the torches scattered throughout. You'll note that this time around you're not being sent back to the beginning of the maze when you don't take the "correct" path. If you are being sent back, then you're here too early and need to progress in the game some more. Here is a handy map to help you navigate the maze a bit easier:



Each dead end has a torch to light. Once you've lit every torch, swap out Flynt for Jack. Now make your way to the line of torches. These get lit as you light the other torches. You can't see them on the map, but it's like, the only other non dead-end that isn't the entrance. You'll find it. Be sure to pick the team you want for the fight as well, Jack is a requirement for the boss. Once you've done all that, you're ready to fight the boss.

Act 4's secret boss is pretty straightforward compared to the last few. You'll want to beat Act 4, then head to the crew zone hallway. Once there, at the far end of the hall is an elevator. Take it down, and you'll find one room with a really bright and obvious door. The guy outside says their room is haunted or smth. Head to the withering woods, and find Minerva. She's that pink Ghost who hangs around one of the graveyards at "Deep Woods C."



Once you find her, it's as easy as buying the pendant she sells and equipping it. Simply head back to the previously mentioned glowing door, and you're ready to fight the boss.

Despite the fact you've beaten Act 4's secret boss, there is still one last boss. You'll notice you've been collecting these gems from all these bosses, and they do have a purpose. First thing you need to do is make sure you've recruited Mizure. Next, you will want to do her 2nd quest. Doing this will unlock a weird tomb in the mountains. At the end of it is a door with 4 familiarly shaped gem holes. Insert each of your gems into the holes at the door, and enter the room. Doing this, you should find a mask in there. Equip it, and head to Poe's caravan. Simply enter the caravan, and you're ready to face the final secret boss.