*Some bosses can be skipped. If you want to open the large door at the end of the game and fight the true final boss, you must defeat all 18 bosses.
The heart in the middle of the sludge is the boss' weak point, but we need to get to it first. Shooting Sludge Heart breaks it apart into multiple pieces. Be sure to focus on the ball that has the heart in it, otherwise you'll make the fight harder by having a bunch of blobs bouncing around the screen. Just keep focusing on the heart until the sludge goes away completely, and then hit it for the kill.
As expected, the Eye Cub's weak point is its eye. If you want a really quick kill, shoot the arrow and then pull it back to hit the eye in the first few seconds. Otherwise you have to be careful of the cube crushing you as it moves around on the grid. If you don't get the quick kill, just be sure to prep your arrow in advance so it's ready to shoot when the eye appears on your side. Just make sure you have enough distance between yourself and the cube before you do it.
You need to remove the ice cube to expose the brain, the boss' weak point. To do so, stand near one of the four switches and move as the boss chases you. It will slide over the switch and activate a flame in the middle. Quickly shoot your arrow over the flame and hit the ice cube to melt it. Then shoot the exposed brain to secure a victory.
Gol-Iath likes to lift its fists and crush the player, so be sure to run and dodge constantly. Also keep an eye out for pink/purple weak spot in the middle and shoot when it's exposed. Like many bosses, prepping the shot in advance helps. If you don't, there's a chance he'll block the weak spot before you hit it.
Second Area (Snow)
Yeti shoots four quick snowballs with its first attack. As it moves around, you'll notice its butt. We have to shoot that butt! After the snowball attack it curls up into a ball and charges the player. Stand near the wall and move out of the way at the last second. Pull back on an arrow and wait for it to land; you should be able to hit its butt with the right timing. If you don't hit it, be sure to dodge the icicles that fall down from the charged ball attack. It repeats the same pattern, so just keep it up until you get the shot.
Stratus emerges from the ground and shoots lasers with its hands, which also emerge from the ground. Run around the large field constantly and keep an eye out on the ground. You can see where the hand will come out in advance. Stand in front of Stratus' face so that his own laser hits his teeth, which protect his weak point. Once one tooth is gone, you'll have a small opening to shoot your arrow and kill Stratus.
Onyxia is one of the coolest boss fights visually, but it's not a particularly difficult one. Keep in mind that you can pull the small glacier platforms by shooting your arrow and retrieving it. Don't stay in the water for too long, as Onyxia will electrocute it after a while. Just stay back and ready an arrow in advance. Let it go when the tail is exposed to kill Onyxia. It's one of those boss fights in which execution is more important than strategy.
Third Area (Forest)
Time to fight a giant mushroom. The screen will turn trippy if you're hit by one of the plumes of smoke. One of them might be poison as well. Obello also jumps around a lot, so you need to constantly be on the move to avoid being crushed. There's a weak point on one of its sides, so this is another fight in which you can pull back an arrow in advance and let it go at the right opportunity (when it lands at the right angle).
Vinethesis' tentacles are very dangerous, but you can shoot them off. Make sure you aim for the base of each tentacle to destroy most of it. Also try to shoot them all off in the same spot so you can roam around the battlefield without running into them (they still kill you even though they've been chopped off). Stay out of the poison cloud and wait for Vinethesis to open its mouth. Shoot the arrow and then pull to open up a weak point. Wait for it to make a full turn and then ready an arrow for the weak point. If you miss, you have to repeat the process again.
Shoot the giant arrow to the left to trigger the fight. Knight Elhanan will charge the giant arrow as his first attack; stand behind a pillar for protection, but be careful. When a pillar is hit twice, it's destroyed. There are only six pillars, so you can't keep the fight going for too long. Elhanan also shoots regular arrows, which are a lot faster. The key is to wait until Elhanan stops shooting and goes to pick up his arrows. At this point, shoot the big arrow to stun him, and then quickly shoot him for the kill. It's a small window of opportunity, so don't worry if you fail to do it the first few times.
You're now fighting a giant mimic, one of the best bosses in the game. Avarice jumps around and shoots giant gold, which can easily crush the small protagonist. It also teleports occasionally, so be sure to move constantly. Its weakness is its mouth, and it will open it to shoot one projectile coin every few attacks. You need to ready the shot in advance to hit the mouth, so recognize the attack patterns and adjust accordingly. If you don't pull back the arrow in advance, you probably won't hit the mouth.
Fifth Area (Fire)
Mol-Qayin shoots out magma balls and pulls them back in. You'll notice its mouth opens to pull back the magma balls, so shoot an arrow right in there. Don't celebrate though, it's not dead yet. Now that it has an arrow in its mouth, you can hold the retrieve button to pull the boss around the environment. You need to pull Mol-Qayin right next to one of the large rocks it shoots out to create an explosion and remove its armor. At that point, quickly retrieve the arrow from its mouth and shoot it while it's vulnerable. Like some other boss fights, it's a small window of opportunity, so don't worry if it takes multiple tries.
Rol-Qayin rolls around at a high speed, so create some distance between yourself and the boss. It leaves behind a trail of fire, which can also kill you if you run into it. Obviously the weakness is the eye, which isn't that hard to target as it rolls around. Just wait for the right opportunity and shoot the eye when you get a chance. This is probably the easiest boss fight in the fire area.
This is a very tough fight. Gol-Qayin will chase you and hit the ground with its giant spike ball hands. If you keep holding the run button he'll never hit you, but we need to get to that weak point on his back. To do this, try to get behind him when you run away. If you're behind him enough, he'll begin a dangerous spin attack. Get some distance, pull back on the arrow, and shoot at just the right moment to hit his back. It's a very tough shot and will likely take multiple tries to pull off. If you mess up, don't stand on the geysers, as he activates them with a big ground attack.
Fifth Area (Graveyard)
Obviously the back is the weak point, but how do we shoot its back when it's so quick? Well, remember that you can kill bosses by pulling your arrow as well. Shoot the arrow away from Gol-Hevel and try to get the right angle so that you can pull it back quickly and hit the weak point. One of the easier boss fights in the game.
Welcome to the easiest fight in the game and one of the few moments of exposition in Titan Souls. Simply climb the Elder and shoot the eye at the top to kill it.
Sixth Area (Final Area)
There are two buttons on the battlefield. Stand on the left or right one and let the fist come down and hit it. Gol-Set's attacks are quick, so don't be surprised if you get crushed a few times before you're able to dodge. Once one button is pressed, go to the other side and repeat the process. Watch out for its laser attacks as you move around. When both buttons have been pressed, Gol-Set's middle area will open up, exposing its weak point. The fists will continue to chase you though, so be sure to dodge and weave as you wait for the middle area to open. Ready an arrow, shoot, and celebrate your victory.
The Soul is super quick and loves to dodge your attacks. You have to roll to avoid its arrow attack. There are also a bunch of electrical currents on the battlefield that you have to avoid. If you miss an attack with your arrow, The Soul will sometimes take control if it and use it against you. Eventually The Soul dodges on his own, and that's your cue to ready an arrow and hit him. This requires a perfect shot though, and you have to anticipate where The Soul moves with its dodge. It took me a lot of tries, so don't worry if you struggle with this fight.
If you've defeated the other 18 bosses, you can open up the final door at the end of the game and fight Truth. When Truth separates in the first phase, ready an arrow for the wheel with the purple eye and let it go when the wheel lands. This will trigger the second phase, in which a bunch of enemies fly around the screen. One of those enemies will be the purple eye, so watch out for it and shoot it to trigger the third and final phase. The eye grows legs and becomes a bigger enemy than can shoot lasers at the ground. The flying enemies are still chasing you as well, so it will be hard to get a shot off. Create some distance between you and the enemies and wait for an opening. Shoot the eye when you get a chance to kill Truth for good.