It will cast Reflect upon itself, then the party, and then will cast powerful magic like Holy, Flare, and Bio upon itself to reflect them to the party. It appears more frequently than other encounters in its floors, and is difficult to escape from (excluding using of Smoke or Teleport).
A good strategy is to use Jump with Kain, physical attacks with Cecil, and maybe throw with Edge if the party has Shurikens. Rydia can reflect spells off the party, and summon. Rosa's healing abilities are limited due to Reflect on the party, preventing her from targeting specific members, or the entire party, but she can cast Holy on a reflected party member to deal damage to Deathmask.
In the 3D releases, the Deathmask still does the above, but also attacks with Laser Barrage, doing upwards of 2,000 damage to the party, enough to kill them in a few attacks. It is vulnerable to Stop, and once afflicted is helpless.
The Deathmask has 37,000 HP, the second highest (first being Zemus's Breath) HP as a random encounter in the game. In the DS version, the Red Dragon has 50,000 HP, and the Behemoth has 47,000, so the Deathmask has the fourth highest HP.