Watcher
The Watcher is the second boss of the game. It watches the Insufferable Crypt. It also watches you suffer, because boy is this boss hard. If you breezed past the Incomplete One and are expecting the same for this boss, prepare to be disappointed.
Preparations
First of all, when you're going to challenge the Watcher, it's best to meticulously explore every single area before it so you can collect as many upgrades as possible. When you get to choose, prioritize health and strength (or skills on the off chance that your run is shaping up to be an excellent skills run). Yes, this does mean it can take almost an hour to reach the Watcher. Yes, this does mean it'll suck extra hard when you die. But the Watcher is extremely difficult compared to everything you've faced so far; you'll need all the help you can get.
Having a shield is awesome here. The invincibility period granted when you take a hit can be invaluable as the Watcher has some attacks that can otherwise hit you rapidly and shred you. It also has several attacks that you benefit greatly from blocking.
First Phase
The fight starts innocently enough. The Watcher will slowly bounce around the room, occasionally stopping so you can actually do some damage. It turns out that the Watcher is pretty annoying to hit! It usually stops at a platform, but sometimes it'll stop below the platform such that you can't hit it while standing on the platform, which is frustrating, but life is frustrating so you'll just have to deal with it.
When its eye turns green, it'll shoot out a wave of eye boogers. Gross, but not too hard to dodge. It's also courteous enough to move away from you before it does this so you don't get shotgunned. Sometimes it'll try to move away from you in your direction, meaning that it'll actually just carry you across the screen before shotgunning you.
If a red line appears between you and it, it's going to charge at you. Roll as soon as you see this and you'll probably dodge it. If you have a shield, you can block it. Doing so will stun it, allowing you to get some precious hits in.
Tentacles: Round 1
When the Watcher is down to 90% of its health, the platforms around the room will disappear and the Watcher will sit in the center of the room. You must then contend with the tentacle assault. Several tentacles will start burrowing through the ground and popping out from under you. If you fail to dodge, it'll hurt.
Dodging these is easier said than done, because you also must destroy every tentacle before you can attack the Watcher again, and while you're attacking one, another could be ready to strike. Keep an eye on the floor at all times; it's important that you keep tabs on every tentacle so you don't get slammed. When one tentacle dies, the Watcher will take a chunk of damage the others will get faster, as indicated by both a color change and, uh, the tentacles being faster.
Second Phase
When the tentacles are gone, you get to fight the Watcher again. One of the chains binding it is gone! This doesn't actually affect its movement, but it's going to start playing rougher. The eye booger attack now includes a second wave that fires through the gaps of the first wave. It also has an attack where it electrifies a circle around it; be ready to roll away if you see this while you're attacking.
Tentacles: Round 2
When the Watcher reaches 66.6 repeating % health, it's time for round two with the tentacles. This time however, things are going to get spicy. Every so often one tentacle will saunter off to the far edge of the room. Remember how I said to keep tabs on all the tentacles earlier? Well, it's really important now. When a tentacle goes to the side of the room, it's preparing to charge at you.
This isn't much of a threat at first; you can easily roll through it or block it. Once the tentacles start picking up speed, though, it becomes the most dangerous thing in the fight. Once their speedometer breaks, they'll be coming at you so fast that it becomes very difficult to react to them while also contending with the other tentacles. It is absolutely vital that you see the tentacle going to the side of the room so you can prepare to dodge its charge. This is a long fight and you cannot afford to get smacked around by every Olympic gold medalist sprinter tentacle. If your roll is on cooldown, remember that you can block Sonic the Tentacle, but not the tentacles that come out from under you.
To mitigate the danger posed by Lightning McTentacle (and the rest of the tentacles), try to reduce the health of each tentacle to almost zero before killing any of them, and then kill them in rapid succession so you don't have to spend long dealing with the fast ones.
Third Phase
Once this round of tentacles is over, the Watcher will start using its signature move, the Ultimate Eye Booger Storm. It'll dart around near the top of the room, rapidly sending waves of eye boogers at you. It's very, very hard to dodge all of these, which is why it's so vital that you minimize the damage you take during the other parts of the fight. If you've got a shield, you can probably stand in a corner and block it all with good timing. Oh, and the regular eye booger attack gets a third wave.
Tentacles: Round 3
Once the Watcher is on its last 25% of health, you'll enter round three with the tentacles. Mercifully, there are no new gimmicks to contend with. Unmercifully, it's just as hard as the second round.
Final Phase, Which is Pretty Much Just the Third Phase Again
When you've gotten a TKO on the tentacles, the Watcher will begin its last stand. All of its chains are gone, and...it still seems to have no effect. It doesn't seem like there are any changes between this phase and the last one, aside from yet another wave of boogers in the eye booger attack. Still, the last one was a doozy and this one will be too. Do not consider yourself the victor until you actually are. Take off its last chunk of health, and it's finally dead; no last-second tentacle tricks. Give yourself a pat on the back unless your back is very sensitive, and...get ready to do it again next time you die...
See below for a video guide to the fight, which shows all of the Watcher's possible moves, and a victory that doesn't rely on any gimmicks you're unlikely to be able to take advantage of.