Paintings - Knife them to play music.
Whomp - Whomp will block your ability to purchase a perk. You can pay him to gain access to that perk machine but he will move in front of another random perk. He originally costs 1,000 to move but the price increases with each purchase.
Bowser Jr. - Start the fight by shooting the cannon and then Bowser Jr.. Regardless of what gun you use, it will always take one bullet. The four cannons will start to move and shoot bullet bills at you. After they shoot, they flip over to show a target. Shoot the target to reveal either a mechakoopa or BJr. If it's a mechakoopa, it will jump onto the stage and explode but you can mostly just ignore them. If it's BJr. shoot him and he will play an animation then disappear. He will only be in one cannon at a time so it's best to shoot both cannons on one side. If he wasn't in either of them, he'll be on the other side. After you shoot him, he will move (or stay) into a random cannon. During this fight, Dry Bones will also spawn. They behave just like normal zombies but only one will spawn at a time per player. (Ex. If you're solo, only one will spawn at a time. If you're playing as 4, up to 4 Dry Bones can be alive at a time). I think it's best to just train them around rather than killing them. After shooting BJr. 3 times (4 if you count the 'tutorial phase'), the boss fight will end. The power switch and a platform will raise up from the lava so you can progress through the map.
Dry Bowser - After you have turned on the power, Dry Bowser will spawn. He behaves like a regular zombie but will sprint for a few seconds if you get too close. The duration he sprints for gets longer each time you trigger him. The first time you kill him, he will drop a power moon that you collect just by walking over it. This will be used as a part for Cappy. Every other time after that, he will drop a random perk. He dies to traps.
Cappy - The wardrobe in the foyer of the castle is where you can build Cappy. There are 3 parts: The Cap is located on the ground behind the power switch. The Eyes are located on the wall of the circular room with the fire spout (second Bowser Jr. portrait on the left). And the Power Moon which you collect the first time you kill Dry Bowser. Placing all three parts in the dresser creates Cappy, which takes a space in your special grenades inventory. You can through him onto regular zombies to turn them. They will distract and attack other zombies. Cappy can't attach to bosses or dogs. WARNING: Sometimes if you are being revived with Quick Revive and you shoot a zombie being controlled by Cappy you can be kicked out of the game for friendly fire.
Boo - Boo will occasionally try to steal one of your perks. You will know he's there because you will hear his laugh, your screen will flash every few seconds, and the icon of the perk he's stealing will flash. When this happens, run to the machine of the perk he's stealing. He can't hurt you and you can't hurt him. Just run up to him to scare him away. After he's stolen your perk or you have scared him, he will fly back to the hallway with all the paintings and enter Ghost Valley. For a few seconds afterwards you will be able to follow him inside (you can enter as long as there are white rings appearing on the painting). After you have killed King Boo, Boo will only spawn one more time.
King Boo - King Boo lives in the Ghost Valley painting. As long as you are in there, he will follow you and try to steal your gun. He doesn't do damage. His health saves so you can deal some damage to him, leave to resupply, and then come back to finish him. As with most bosses in this map, his health scales with the rounds so the later you try to kill him, the higher his health. After you've killed him, Boo will only spawn one more time, the painting will remain permanently open for you to travel between worlds, and a platform will rise which allows you to Double Pack a Punch.
Wario & Waluigi - Occasionally, a round will be replaced with Wario and Waluigi. They spawn from a pipe at the spawn area. They are very fast and have a lot of health but will not damage you. If they catch you, half your points will be taken. Your points can be stolen by each person so if you have 100 points Wario will reduce you to 50 and then Waluigi will reduce you to 25. Killing them will return your points. Regardless of whether you kill them or not, they will drop a max ammo (if you don't kill them it will be at the spawn area). They die to traps.
Pack a Punch - The first time you try to enter PaP, Magikoopa will appear and remove the bridge. Walk through the map to find and collect all the pieces, 7 in total. The bridge will complete itself as you find them so once you find the last one, PaP is available.
Bowser - After completing the PaP bridge, a bowser platform will appear in the spawn area. If all players are standing on it, a MarioKart race between you and Bowser will begin. During the race zombies can't kill you and you can't kill them. If you lose the race, nothing happens. If you win, Bowser will spawn as an enemy. (Remember the race ends for all players once first place completes his last lap). Bowser has a lot of health but is also the only boss that gives points so he's very good to farm. If you don't care about the points, you can kill him with a trap. If you want him to respawn you can race him again.