Super quick and dirty guide on math for Construct 7.
When he casts 'Subtract', he'll bring your HP down to a single digit - make a note of what this number is. He'll also spawn 4 puddles on the ground in a line that are denoted with either 1, 2, 3 or 4 dots. Going into the puddle area will ADD that the dot amount to your HP. The main mechanic is to math and make sure that your HP will match the mechanic he's doing.
'Divide by 3'. You need to make sure that your HP is divisible by 3, so 3, 6, 9 or 12. Go into the puddle that will add points to your HP to hit one of those numbers.
'Divide by 4' - Your HP needs to be an even number, 4, 8, 12, 16 etc.
'Divide by 5' Your HP needs to end in a 5 or 0.
If you do it correctly, you'll see the check mark on your head and you'll get a damage up buff.
After the cyclone, you'll be doing advanced math. He'll cast 'Indivisible', which means your HP needs to be a number that cannot be divided (aka a prime number). 2, 3, 5, 7, 11, 13.The FFXIV playerbase does not hold out much hope for this fight. It's amusing that this is a mechanic which would be completely ineffective in WoW. An add-on would be immediately made which gives you the correct answer right away.