Deathmask will start off casting Reflect on itself, and then every party member. It will start casting Holy, Flare, and Bio on itself and every now and again casting Curaga on the party to heal itself. If it comes with a Full Body, it will cast Reflect on its companion in addition to itself, and later on cast Haste on the party, ostensibly to reflect it onto its own side.
The best way to handle a Deathmask is to cast Black Magic on the party due to the Reflect spell, or use Rydia's Summon ability. X-Potions can be used for healing to get around the Reflect status. If Edward Chris von Muir is in the party his Salve ability is excellent in the most critical of situations.