I am kind of obsessed with the bosses in this game, so I'm going to take a little time to give a walkthrough for all the hard mode bosses.
Read these if you are having trouble with any of the bosses, or if you just want to reflect on strategy!
This boss is vulnerable at its core, but you must blast away at the inner shielding to get through. Do not shoot too much when the Silencer is moving, as the shots will be deflected back towards you. Watch for its erratic movement, as it may crush you against a wall. Wait until it comes to a standstill, dodge any bullets, and pummel the shielding/inner core with bullets until it starts moving again. Repeat until the boss is no more.
The hydra has many heads, and the only way you can hurt it is by shooting the open mouths. Fire bullets constantly, and stay near the top of the screen to be safe, as the heads fire a greater amount of bullets lower down on the screen. When you destroy a head, another one will appear, shooting falling bullets (another reason to stay high up). When you destroy that head, one will sprout that shoots giant lasers. After that, the heads die off. 3 heads x 3 incarnations = 9 enemies to kill. Keep moving, staying high up, and don't worry about the flying torpedos coming from the back, as they will be taken out by your fire. Once you destroy all the heads, this boss is history.
If you haven't leveled up your gun enough, this can be a very taxing battle. The reaper drone goes through 4 phases of spawning enemies. They will stay on the screen until the 4th phase, where he is most vulnerable. During the 4th spawning, give the boss drone all you've got, especially when it stands still for a moment. It will then repeat its phases. Watch out for phase 2, as the drones spread out, hitting a wide area. Be nimble and try to skim the outer edges of the screen, since the drones follow you. It should give you slightly more room to maneuver. Focus on dodging until the boss' 4th phase, and hit him during that time until he dies.
The Molten Processor is invincible until you destroy all the tentacles that are rising from the lava. Watch out for lots of incoming fire, and take out the tentacles. When you do, the boss' defenses will lower, and you should shoot the bars in the front to "open" the boss. After a very short period of time, the tentacles will respawn and the boss' defensive orbs will come back. The next time you lower the defenses, you can get inside the boss, and fire as many bullets as you can into one or two of the boss' 4 cores before the defensive orbs come back. Repeat the process of lowering the defenses and getting inside until you've destroyed all the cores, defeating the Molten Processor.
Make sure your gun is level 3 before entering this battle, as it is a test of your speed and endurance. The battle starts when you enter the center of the boss, and you will not be able to leave until it is defeated. The Rock Smasher has 8 cores to explode. Be firing constantly, and do your best to avoid incoming fire horizontally, vertically, and from weird diagonals. If you are good enough at dodging, you will be able to get a lot of hits on the cores and take minimal hits yourself. Notice that as you destroy the cores, you will take slightly less fire, but don't let your guard down until you've destroyed every part of this boss.
This is probably one of the toughest bosses on Hard mode, because of the sheer amount of enemies that will be spawning on the screen. When you enter the battle, the Grand Mother will immediately birth two smaller "Mother" ships, which will be constantly birthing more smaller ships. Focus on destroying the two Mothers, because they are the Grand Mother's weakness. Watch out, because each time you destroy a mother, the Grand Mother goes nuts and starts spewing bullets in a spiral pattern. You will need to destroy 4 Mothers (2 rounds of 2 Mothers) before the Grand Mother is vulnerable. Defeating the Mothers is the hardest part of the battle, because there are enemies everywhere. If the Mothers are too close to the Grand Mother, focus on killing the tiny ships until you can get a clear shot on the Mothers. A little luck is involved, concerning where the Mothers move to. Once you destroy 4 Mothers, it's smooth sailing, as all you need to do is fire bullets into the Grand Mother's core (from both sides), while dodging the small amount of bullets it fires. Don't worry if you only have a small amount of health at the last part, you should be able to win if you have moderate dodging skills.
The guardian is vulnerable at its waistline, where its core is in view. It will jump back and forth across the screen, while going through 3 firing patterns. 1: It will fire a bunch of small bullets outward. 2: It will launch 3 huge cannonballs at you; move clear out of the way. 3: It will fire a huge laser straight out from its head; this is not dangerous if you are shooting its core. -- After taking about 1/3 damage, the two stationary soldiers at the top of the screen will become "alive" and start homing in on you. Fly up high and deal with them, making sure to dodge Guardian's attack #2 if you see it coming. After another 1/3 damage, the two large bugs (I forget their names) will become alive. Fly up high again to destroy them before going back to the Guardian, and watch for their lasers and other fire, which goes all over the screen.
Once you deplete the Guardian's health, it will go into a second boss form, where its upper and lower half split apart and start jumping and flying around, respectively. When they do this, the core in the middle will start flying around - keep your eye on it! The core, which is somewhat hard to recognize from the other bullets, is the only vulnerable place to attack. Follow it around, and watch out for the jumping lower half and the flying upper half of the Guardian, which shoot sporadic bullets at you. Once you sink enough bullets into the core (about half the amount it took to kill the original form), the Guardian will be obliterated.
This boss presents quite a nice challenge for those who are not adept at dodging lots of different things at once. The Star Splitter has 8 cores to destroy; the instant you enter the room you should start shooting the top one, or use the level 3 blade on it. The boss will immediately sprout 3 spiked arms, which attach onto the walls. For the duration of the first part of the boss fight, the Star Splitter will launch 2 arms onto different walls. Each time an arm hits a wall, rocks will shower down from the spot where it hit. If the arms glow, it means they will bounce a few times before showering rocks below. If you have little space to move, stay underneath the impact spot, as the rocks will go on either side of you, not causing any damage. As the boss launches its arms around the room, watch out for incoming fire, as well as the midpoint of the arms where there is an invincible orb. Try to keep a good distance from the main body and the arms, because you don't know which way they will move. Keep firing at the cores while keeping an eye on all the movement on the screen. The lower corner cores are particularly hard to hit; don't become overcomfident just because the boss' health bar is low.
When you explode all 8 cores, the Star Splitter's main body starts flying around, bouncing off the walls and shooting everywhere. At this point, the boss is vulnerable anywhere on its body, so avoid being hit by the bullets and especially the main body, while assailing it with fire. Once you destroy this part, the Star Splitter is gone for good.
This boss consists of two small flying guys who move and shoot in a similar fashion to the player. Don't be fooled by their small size; they have an incredible amount of health, and since there is two of them, they can flank you easily. The "Elite" guys will be shooting level 1 bullets constantly, and they move really erratically, so you'd better be on your toes. Any time they stand still for more than 1 second, they are charging their "spread" fire, and will release a huge ring of bullets a few seconds after they start moving again (similar to the player's "expel" power). They always do this at the same time. The bosses also have a tendency to try and slam into the player, causing both people damage. The best way to handle the fight, overall, is to move as erratically as the bosses. If you stay still or move predictably, they will home in on you. Once you defeat one Elite, the fight becomes a bit easier, but do not let your guard down until you're the last man standing.
This guy, labeled a "hunter-killer" shows up every once in a while in Hero Core, destroying the other on-screen enemies to have a 1-on-1 battle with you. You should avoid battling him until you have the level 3 gun. When you do try to take down the Eliminator, take him on only in a large room. He can fly through walls, and if you don't kill him in time, he will run away, to appear randomly another time somewhere else (he can appear almost anywhere). To destroy the Eliminator, you must study his movement pattern, and try to move upwards and downwards with him as you shoot all your bullets into him. If you miss too much, you won't damage him enough in time. Killing this optional boss takes patience, and only yields a small level up, so save him for last (before Tetron, of course) if you want.
Tetron can be easy to beat if your previous exercises in dodging bullets have payed off. The final boss has three forms:
In the first form, Tetron will fly slowly around the top of the room, taking turns shooting homing laser-bullets, and huge rings of cannonballs and the like. Focus on dodging the laser-bullets, while peppering your own bullets onto his head, the only place where Tetron is vulnerable. He often will move from one side of the room to another, so try to make sure you are not pressed against the wall, where it is extremely hard to dodge oncoming bullets. When you deplete his large health bar, Tetron goes into his second form.
In the second form, Tetron will shoot more bullets, but they will be less powerful. He moves around a bit more here and occasionally fills the room with lava (not an issue if you are immune), so be careful. His weak point is still is head, so fire on it as much as you can without being hit by the bullets flying everywhere. This form only has half a health bar.
In his third & final form, Tetron will land on the ground, with his gun raised - this blocks his head, preventing him from being hurt. At this point he will cycle through three phases - 1: He will shoot a bunch of laser-bullets, first a spread, then a few homing ones. 2: He will launch a ton of smaller spreadfire bullets. 3: He will fire huge cannonballs, while lowering his arm. -- During #3, when his arm is lowered, fire as many bullets as you can into his head before he raises his arm up again. You will have to dodge the same flurry of bullets each time, so if you are used to dodging them, you should have no problem defeating his last form. Once you have depleted the last half-health-bar, congrats, you have beaten one difficulty mode of Hero Core, and can treat yourself to a satisfying ending!