Please pay attention, this skills are explored by keyboard in Nightmare, i.e., if you have a gamepad, it could be only easier for you to perform these skills.
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Before we begin talk about the skills, please let me comment some words of nonsense. I think King of Floor (Fact) is the most gorgeous and rare BOSS in the history of ARPG. In Ys, some BOSS needs your skills, like the bat and the mantis; Some BOSS are very hard-core so that you could do nothing but improve your ability, like the two heads. However, King of Floor could be a most high-level BOSS who needs both your skills and your ability.
Therefore, the only thing that I could do here is just share my skills. Because King of Floor is skillful with removing the floors, many adventurers are easy to be involved in a dilemma of unable to notice the route of the BOSS when concentrating on the order of removed floors. In fact, if you want to overcome this BOSS, you need to remember 4 critical tips:

That means, because only when your route coincides with the BOSS's route could you make a large amount of damage, you should not notice any details, including floors under your feet and the bomb from the BOSS, other than the route of the BOSS. The experiments show that the coincided routes should not be too long, because a too long attack could divide your walking area into 2 parts, which is what you do not want to get; And you should not make the coincided routes too short either, because the damage is too low. Some people suggest that we should strike the BOSS in the opposite direction, which is totally useless in a Hard or more difficult mode. Therefore, the only thing you could do is just keeping pace with the route of the BOSS, and giving him a not too long and not too short heavy attack.

Because of the 1st tip, you have no energy to notice the bombs of the BOSS. However, you still have a method to avoid them. Because the bombs are always tracing where you stand currently, you have to keep running. Besides, because the bombs could be split into 8 smaller bombs in 8 directions, if you always run in the same direction (like left-right direction or diagonal direction), your running would be definitely useless. Therefore, the real effective method of avoiding should be keeping turning directions. The most idea condition is that you have the enough space to move in a circle; But with the battle gets severe, your moving space would be smaller and smaller. In this condition the best route is keep changing your routes in different crossroads which are built by yourself, so that you could avoid running in the same direction. Noted that the above skill is totally based on marco-thinking, thus you still need to use this method to get close to the BOSS's track.

Another time, because of the 1st tip, you could not be aware of the floor under your feet. So you should have a order of removed floors in marco-thinking. The main tip is, remove the 4 corners at first, after that try to remove 4 edges of your moving area. If you could master this, you could give the BOSS a first heavy attack. And then you may could have the skill to repeat this process with the current moving area, although in practice you could only remove 2 edges, but it does not matter. If you could do this, congratulations! Now you could shrink your moving area slightly and slightly. When BOSS is nearly dead, you could omit this tip and give as heavy damage as you could. Noted that you still need to view the 1st tip as the most critical task. Therefore, this tip gives you a suggested position of attack the BOSS actually.

Do not keep moving in a small space because you want to attack the BOSS, this is a really dangerous action. One reason is that it is easier for you to get attacked by tracking bombs. Another reason is that you could be easy to trap yourself in a deadly space because you remove the surrounded floors. Therefore, even if you remove the left part of the floors then remove the right part, you could get suffered by such a ridiculous decision. Because this decision means in more than half of the whole area you could not touch the BOSS at all. So you should keep cautious so that the walking area could be spread over the whole area.

If you understand and master the 4 tips, you could practice repeatly. I believe it will be easier for you to approaching the success. If you could manage to perform the master's skills, in theoretical you could keep 1/2 of your HP when the BOSS only have 1/4 HP, which makes you have enough HP to omit any damage but concentrate on give the BOSS a final attack in such a small walking area.