Boss Guide (Part 1)
*Denotes bosses new to the Rebirth version.
The Basement/The Cellar
Dingle is a giant piece of poop. Lovely. He only has two attacks: a triple shot and charge attack. He also summons addition poop squirts, in which he whistles as a warning. He then immediately does a triple shot after the summon, so dodge it. Also be careful when he charges, as Dingle can move very fast. Don't be surprised if you get hit once or twice. After he's done charging, Dingle loses his breath and gives the player a great opportunity to attack. Keep up with this cycle until you defeat him.
The Duke of Flies
The Duke of Flies is easy. The only problem is he spawns a ton of flies, and at times you can lose track of them all. Just be sure to kill all the flies first and then fire tears at him. At times it will be hard to hit him because of the flies surrounding him, but stick to the strategy. It's one of the simpler boss fights, but you can make mistakes if you're not careful.
You have to beat the game once to unlock Famine, one of the four Horsemen of the Apocalypse. He likes to charge through the screen and pop up on the other side, so be wary of that attack. He also spawns additional enemies: Pooters and Fat Flies. Get rid of additional enemies before you set your sights on Famine again, and continue to keep an eye out for the charge attack. As long as you see it coming, you can easily avoid it.
When he loses most of his health, the body and horse disappear. Only Famine's head remains, which shoots triple shots at the player. Avoid those and continue to shoot at Famine until he goes down.
These two conjoined twins will follow you around the level, while the small one shoots. Eventually the big twin will get a speed boost and then need to catch his breath - this is the perfect opportunity to attack. Otherwise, run around the edges of the room so you can keep your distance and still shoot the boss. The twins can be killed in either order, but it's best to get rid of the small one first. If not, he will follow you around at increased speeds and can be hard to avoid.
The Haunt is surrounded by three small ghosts. You have to kill the three ghosts first before you can actually damage The Haunt. At this point he starts moving left and right at the top of the screen and either shoots a brimstone laser or red projectiles straight ahead. You can get on his sides to avoid the attacks, but he will also quickly charge left and right if he knows you're there. Alternate between his sides and front to attack, and avoid the brimstone laser at all costs.
Stay away from Gurdy Jr. He has a fast charge attack that comes without warning and poses the biggest threat. He also spawns additional enemies and shoots blood in eight directions. Take out the enemies before you focus on Gurdy Jr. again and remember: stay away from him. Don't be surprised if he charges straight into you a couple of times, even if you're doing a good job of staying away.
The Gurglings boss fight is about execution more so than strategy. These two guys simply move around the level and charge at the player, leaving behind a dangerous trail of blood. They also spawn flies occasionally. That's it. The danger is the charge move, so don't get too close to the Gurglings. Focus on one at a time, just like any boss of two or more.
Larry Jr. is comprised of two segmented worms, and players must destroy both to defeat the boss. Larry Jr. kind of does his own thing and moves around the level, periodically leaving behind poop. Get rid of the poop so you don't trap yourself, and always be on the move. Larry Jr. won't become aggressive unless he sees you head on. Stay away as much as possible and shoot from across the room. Also, focus on one worm at a time to make your life easier.
Another easy boss fight with easy to telegraph moves. He either hops at the player, shoots blood at them, or jumps high up into the air and lands, shooting blood in all directions. Defense is the most important thing here, so just pay attention to his attacks and where he lands. You should be able to kill Monstro without getting hit if you dodge and shoot effectively. There's not much strategy beyond that.
Pin dives up and out of the ground periodically, and also shoots four shots in a X pattern when it goes back in the ground. The shots are slow and easy to avoid though. Then it pops out of the ground in a single spot, which is the perfect opportunity to attack. At that point it will throw out an explosive shot before going back in the ground. Again, it's easy to avoid, so don't worry too much. Just move around constantly so Pin doesn't come up out of the ground when you're right on top of him.
This upside down spider constantly jumps around the room and creates white Sacks, which spawn spiders if you don't destroy them fast enough. It also leaves white puddles on the ground which slow the player, so don't step in the liquid. Sometimes it's best to ignore the Sacks and just focus on damaging Widow, because killing smaller spiders doesn't take too much time. Widow covers a lot of ground, so be extra careful when it jumps around.
If Chub has a direct line of sight to the player, he charges. Avoiding the charge isn't a problem, but you should drop a bomb before you move out of the way so Chub eats it. The bomb does a ton of damage and speeds up the fight considerably. Chub will also summon other Chargers to help with the fight, so take them out while you move around and shoot Chub. Overall, this is one of the easier boss fights.
The Dark One*
The Dark One has three main attacks: a charge, an 8-way shot, and a Brimstone beam if you happen to be standing in front of him. Additionally, The Dark One can appropriately darken the room and make it hard to see. Don't stand in front of The Dark One to eliminate the threat of Brimstone, and keep your distance to better avoid the 8-way shot. It sounds easy, but The Dark One reacts quickly and proves to be quite challenging.
This fight is all about crowd control. Fistula breaks into smaller pieces the more you attack it, so you definitely want to focus on one piece at a time. The parts bounce around, collide into each other, and change course. The key is to maneauver around all the pieces and not get hit. Again, try to focus on single pieces so you're not running around with a million things flying on the screen. It's a lot easier said than done.
Gurdy stays in one place but summons additional enemies (flies) and Boils in front of her. Get rid of the enemies and Boils when you see them, and then focus on her. She moves her head to the side when she's ready to shoot projectiles at the player, so use that tell to your advantage. Shift back and forth between her sides and front to shoot at her, and keep getting rid of additional enemies when she summons them. This fight shouldn't cause too much trouble.
This is basically the Larry Jr. boss fight again, except that The Hollow can move diagonally. Employ the same strategy as Larry Jr. and be careful about getting stuck on rocks (if the level includes them).
The Husk acts like Duke of Flies, but summons additional spiders and has actual attacks, like a blood shot that covers four directions and the spit attacks similar to that of Monstro. Flies aren't a problem, but you definitely want to kill the spiders immediately. His 8-way attack can be tough to dodge, so keep your distance to give yourself time to move out of its way. The Husk doesn't have a lot of health, so the fight won't take long.
One of the most annoying bosses because of how quickly he jumps at the player. There's no indication it's going to happen, so you can't truly prepare for the attack. Luckily he focuses more on his vomit attack, which you can avoid by looking at the shadows on the ground to see where the vomit will land. He also shoots projectiles and spawns Squirts, but the aforementioned jump is the biggest threat.
Mega Maw stays in one plays and usually shoots out purple flames in various patterns. Because he stays in one place, your best bet is to stick to the bottom of the room so you can avoid the purple flames. Mega Maw also summons hoppers, which you should kill quickly. Lastly, Mega Maw shoots blood much like Monstro, so move side-to-side to avoid it. The biggest challenge here is when Mega Maw summons hoppers and then attacks. It can be hard to focus on both, but do your best. Your priority should be the purple flames over the additional enemies.
Peep will either shoot out urine in all directions, jump and land where you're standing, or release yellow liquid that damages you if you stand in it. All of these attacks can be avoided with relative ease, but as you damage Peep he loses his eyeballs and they move around the screen. Trying to avoid the eyes and Peep's attacks simultaneously is where the real challenge kicks in. Don't bother shooting the eyeballs as it doesn't hurt Peep. Instead, focus on his body and do your best to dodge all attacks. This is another one in which execution proves more important than strategy.
You have to beat the game once to unlock Pestilence, one of the four Horsemen of the Apocalypse. He fires green shots that explode on impact and leaves green liquid on the ground that can damage the player. He also summons additional Chargers and Spitties. Once you've done enough damage (half), his head disappears but the body and horse remain. His moves are the same, but the explosive shots are a lot faster. Move around at all times to avoid getting hit and make sure you kill additional enemies that are summoned before dealing with Pestilence. As long as you don't get overwhelmed by enemies you should be able to handle this fight without too much trouble.
A harder version of Widow, in which she spawns two spiders, a big spider, or a Trite. In addition, the white puddles are bigger in this fight and she can shoot white projectiles in eight directions. Use the same strategy as Widow but focus on defense more so than anything else.