Venoxis comes with 4 snake adds. He stays in priest form to begin where he cast Holy Nova, Holy Fire, and Renew. He can also cast Holy Wrath, a spell which jumps from person to person with damage increasing exponentially as it hits people. In snake form his melee damage goes up dramatically, and he releases a poison cloud.
Strategy
The OT must pull the adds to the rest of his raid, killing them one at a time. Sheep 3, sleep 3, blind, trap, whatever you have. The point is to have only 1 fighting at a time. Take them down quickly and move onto the next one, allowing the OT to break CC. Do this so the Priest cant use his chain lighting to hit even people far away. This can whipe a raid.
Next you all run in to where the MT is tanking the boss. He should be tanked near the bonfire for best results, I've seen. When the snake adds are dead, get within shooting range of the boss and unload anything you want on him, while conserving some mana (meaning wands, bows, dots, low mana cost spells).
If he releases a cloud of poison the MT has to move him arround to avoid taking too much dmg, just keep ranged dps up till hes down.