Masq is, to put it simply, a dance first and a boss fight second. Learning that dance, and how to move with it while attacking, is the key to succeeding in this fight. Aside from the advice listed above, I've got a few tips for the survival aspect of the fight.
Further in, you'll start to notice a healing reduction effect, and Masq's attacks will start inflicting the "injury" status, which cuts your max HP. However, curing injury with a Sol atomizer will instantly replenish all the cut HP, even with the heal reduction in effect... use this to your advantage! Sol atomizers will actually be better for healing your HP than mates/Resta/megiverse will, and if you have an Atomizer lovers ring, you'll actually be invulnerable while using an Atomizer item. That can grant you a very much needed moment of safety when you need it most.
Also, poison immunity, whether through a skill ring or timed abilities, is a must-have imo. None of the skills that protect you from dying (and leaving you at 1HP instead) work when you're already at 1 HP... which the combination of poison and injury will tend to do more often than not. So yeah. Poison kills. A lot. Make sure you've got a plan to deal with it.