I'm talking about Magery Mastery on the character, not the pet. Death Ray damage is based on Magery, Eval, and Mastery Level. It's NOT affected by SDI. It deals Energy Damage, so anything that reduces the opponent's Energy Resist will buff Death Ray's damage output. Death Ray itself also lowers the victim's Energy Resist by 10, stacking with Disco/Rune Corruption/Conductive Blast, even into the negatives. When Pub 100 goes live, for a player wielding a Super Slayer spellbook, Death Ray will deal +100% Damage to the appropriate target, and for a character wielding a Lesser Slayer spellbook, Death Ray will deal +200% damage to the appropriate target.
On a char with 120 Magery/120 Eval/Lvl 3 Magery Mastery and using a Dragon Slayer spellbook, i've done 530+ damage Death Ray ticks to a discorded Greater Dragon that was under Conductive Blast. It's basically WoD level damage at any health, and ticks every 3 seconds.
Death Ray has some limitations though. It's very mana intensive (50 Mana on initiation, 35 per tick, subject to LMC) if you move it cancels the channel, if you get hit it cancels the channel (why tank is important), if you use any item (even in your pack), it cancels the channel, and only one Death Ray can be active at a time on a single target.
A good template for it, is the good ol' 6xLeg Tamer/Mage template. 120 Taming/120 Lore/120 Vet/120 Magery/120 Eval/120 Med. Since this template lacks Discordance, you'll want Discordance on your pet to further buff Death Ray damage. Rune Corruption and Conductive Blast will also boost Death Ray damage. Only a few pets are capable of learning both Discordance and Conductive Blast at the same time, the foremost being the Cu Sidhe and Unicorn. The Cu Sidhe has the advantage of being especially tanky and being able to self heal with bandages, but it's going to be wasting time and Mana on Bleed attack. The Unicorn is less tanky than the Cu Sidhe, but is completely immune to the Poison status, and can be trained to know nothing but Discord and Conductive Blast, so once it gets Discord off, it'll spam Conductive Blast, giving it more of an uptime vs the Cu's CB uptime. Disco+CB Cu=Tankier, Disco+CB Unicorn=More Damage.
Selecting Magery Mastery for Death Ray on a Tamer/Mage means you can't use Combat Training: Consume Damage while Magery Mastery is active, so you're basically trading pet defense (Consume Damage) for more offense (Death Ray). Although, the best defense is a good offense...