(The above video uses captions for explanation. Please turn English captions on.)
The Skin Splitter, the boss of Water Dragon Sanctum, is a common sight throughout most of Elrianode. With this boss appearing so frequently (especially later on, where you will have to deal with him alongside several other threats), knowing how to handle the boss' moves can be invaluable for keeping out of danger and maximizing your chances to safely attack. I hope the resources below will help clear up any mysteries about the Splitter, and aid you on your quest to save the El~.
The Skin Splitter is a massive juggernaut that will rarely react to your attacks.
Overall, the Skin Splitter has fairly simple behavior. The Splitter will choose a target and attack to use. If the Splitter thinks it won't be able to hit the target from where it's standing, it will walk closer to that player before attempting the attack.
Note that, in any location where a Skin Splitter can be fought, there are platforms nearby. These can be powerful tools in evading the Splitter's moves; for this reason, it's helpful to get the Splitter to follow you near these platforms. Having these ready can give you a lot more mobility and options for trying to dodge.
The Skin Splitter will indicate its Circle Sweep attack swinging its enlarged arm to its side. Afterwards, it will spin that arm around, covering both sides of it and sending any player hit by it flying. The Splitter will take a few moments to stand back up after performing this attack.
An immediate warning I must give about this attack is that the sweep's hitbox is far taller than one would think. Strangely, a person directly above the Splitter (in the air or on a platform) would be clobbered by the sweep. This makes using height to avoid the attack unusable, and going over or behind the Splitter during the sweep unnecessarily dangerous.
The best course of action to take with this attack is to simply get some distance from the Splitter, and return once the Splitter has finished spinning. The Splitter takes a bit of time to recover after using the sweep, so any player who jumped out of the way will be able to smack the Splitter a few times before it can take another action.
Alternatively, a player can also consider using a special active during this move, using the I-frames to avoid the spin and end up right next to the Splitter as it finishes the Circle Sweep, giving that player a bit more time to punish the attack.
Using super armor against this attack is NOT recommended. The reason is simple: the Circle Sweep actually hits several times. However, most players will only get hit once or twice by the attack because the attack launches players. This launch is what gets players out of the sweep's hitbox, keeping them from getting hurt further. If you have super armor, on the other hand, you're probably going to take every hit and lose twice the amount of health you would have without super armor.
The Skin Splitter will prepare its Slam Attack by bringing its enlarged arm behind it, slowly raising it. It then brings this arm from behind it to in front of it in an overhead swing, smashing anyone in front of it.
Note that the path the Splitter's arm takes involves starting behind the Splitter, passing over the Splitter's head, and then hitting the ground in front of it. Because of this, attempting to jump over the Splitter during the Slam Attack is futile, and almost guarantees that you'll get sent flying backwards. This also makes attempting to get behind the Splitter hazardous: while it is possible to get out of the reach of the attack's backswing, you're also just as likely to fling yourself into the backswing if you aren't quick enough (which is just as painful as getting slammed in front of the Splitter).
If you happen to be a fair distance away from the Splitter, it's easiest to simply back away from the Splitter and let it finish its attack. There's not too much that could be done at this range to punish the attack without resorting to special actives or some other course that could get you hurt.
Curiously, if you're next to the Splitter, the safest place to be is right in front of the Splitter. Likely owing to how the Splitter moves its arm in an arc, there's a spot right next to the Splitter where the Slam Attack will not hit a player. This means a player can stand right in front of the Splitter, keep attacking with commands or actives, and be completely safe. However, it must be emphasized that this is a small spot. If you jump, you'll hop right into the slam's arc; this spot can also fail if you are dashing into the Splitter, probably because of your character's hitbox being moved. If you really want to make sure that the spot will work, simply stand still and the arm will pass harmlessly over you.
The Skin Splitter will put its enlarged arm on the ground to signal the beginning of its Football Charge. After a moment, it will begin running forward, continuously hitting and carrying along any targets hit. If the target cannot be moved, the Splitter will end up running in place. After a couple of seconds of running, the Splitter will fall down, resulting a strong final hit that knocks hit targets backwards.
The Skin Splitter will also emit a low growl while entering the initial stance for the charge; if the Splitter is offscreen, keep your ears peeled for this, as it's a sign that you may need to get out of the way. The Splitter runs at a speed faster than most characters can run, so attempting to outrun the Splitter is often going to end badly.
During this attack, the Splitter's entire body is a hitbox. This means that you will get hurt by the charge if you touch the Splitter in any way - even if you touch the top of the Splitter (which can happen if you're on a platform) or touch its back while it's charging (which can happen if the Splitter is running into a wall).
There are two effective ways of dealing with this attack. The first option is to use a combo or move to pass behind the Splitter before it starts running; the Splitter will harmlessly run in the other direction, away from you. However, this may not be possible if the Splitter has its back to a wall when it gets ready to run.
Alternatively, if you have nearby platforms, you can make use of them to hop over the Splitter while it is in its ready stance, or as it runs under you. This can completely avoid the charge with a single jump.
Do NOT attempt to I-frame through the Splitter's Football Charge. Even if you're using a special active, in most cases (there are some exceptions), the Splitter will still have its path blocked by you and run in place. This will result in you getting creamed the instant your special active is over. This may mitigate the damage, but the other two options completely protect you, and don't have nearly as much of a cost.
Lastly, be careful of what summons you have out, as they can block the Splitter and force it to run in place. This naturally includes summons from skills (e.g. Code Empress' Oberon and Ophelia), but can also include summons from Secret Dungeon equipment (such as the Type-H summon from the Alterasia set). Keep an eye on what will be in the Splitter's path, try to see where the Splitter might get stuck, and react accordingly.
The Skin Splitter will begin its Henir Blast attack by gathering dark energy around its enlarged arm, stretching it forwards. Shortly after, the Splitter will shoot a burst of energy over a very long range, placing any players hit into a lengthy hitstun. After this burst, 5 bombs will float into positions over the burst's range, and detonate shortly after; any player hit by the initial burst is almost guaranteed to be hit by bombs. Each bomb is individually capable of dealing damage. After the Splitter fires off the burst, it will collapse to the ground, unable to move for several seconds.
The range covered by the Henir Blast is massive, stretching well beyond what a screen can display. Because of this extreme range, outrunning the Blast is nearly impossible for all except for a select few characters.
Going on a platform above the Splitter to evade the Blast is possible, but risky. Both parts of the Henir Blast - the initial burst, as well as the bombs - have lingering hitboxes that last for a couple seconds longer than one might believe. Because of this, it's possible to dodge the Henir Blast at first by going above the Splitter, only to descend too early and jump back into the attack's hitboxes. It's also often quite difficult to attack the Splitter from such a position, wasting the chance to take advantage of the Splitter trying to recover.
Since this attack does not hit behind the Splitter in any way, the best course of action is to get behind the Splitter, be it through a pass-through combo or active, or by using platforms to jump over the titan. With the Splitter resting after the attack, it's a prime opportunity to rack up hits on the giant.
If the Splitter happens to use this with its back to a wall (making getting behind it nearly impossible), using a special active to get I-frames is also an excellent choice. Doing this will get a player past the Blast's initial burst. Since the bombs don't detonate near the Splitter, this also means that a player that does this will be able to attack the Skin Splitter freely.
Be wary of this attack if the Splitter is nearly dead. If the Splitter dies with Henir Blast bombs active, the bombs will detonate immediately. This could potentially cluster the bomb explosions together and deal severe damage if you aren't in a safe place or state.
The Splitter only moves if it lacks a target, or uses the Football Charge. I've gone over the initial luring and how the platforms can help out. However, it can also be helpful to think about where the Splitter can end up after charging. This is something you'll likely have to weigh on your party's needs, but if you stay to one side of the Splitter, you're likely going to get the Splitter to charge in that direction. Play your cards right, and this means putting the Splitter next to platforms, or possibly a wall.
A wall can have mixed effects in this fight: it would certainly help some classes deal the most damage they can (such as an Eve with Dimension Link - Guardian) and give many characters a chance to get extra hits on the Splitter as it runs into the wall (especially ranged characters), but also puts in a couple new obstacles (with Henir Blast, you'd be forced to either go above the Splitter or I-frame through). Consider your party carefully in order to figure out what would be best for the group.
The Skin Splitter's Circle Sweep attack makes MP management an important concept to consider. Without a special active, while you can get a couple of hits in after dodging the sweep and returning, it's not a lot. The Sweep slows down chances to attack more than any other - for this reason, it's probably the attack people would want to use a special active against the most often.
The Henir Blast can potentially fall into the same situation if the Splitter has its back to a wall. In this case, only a special active would allow a player to both dodge and have time to punish the move. Outside these situations, though, getting behind the Splitter is easy enough, and allows plenty of time to farm MP off the Splitter while it tries to get up.
The Football Charge doesn't give you a lot of room to punish the attack, given the Splitter's high speed and body-wide hitboxes. This naturally changes if you happen to have a place in which you can catch the Splitter near the end of its run with a special active, or if the Splitter is running into a wall. Whether you ought to gain MP off this move, or try to use skills to punish it, likely depends on the situation you happen to be in.
The Slam Attack, assuming you are in front of the Splitter, requires absolutely no reaction from a player. A person can just keep attacking and gaining MP while the Splitter uses this without issue, just so long as he or she doesn't dash.
A sample solo battle against the Skin Splitter in Water Dragon Sanctum can be found here. I invite you to use the facts in this guide to watch my actions, seeing why I did what I did, and where I may have made mistakes.