Well if annihilation is your Main speel goal you gotta bump his Dmg up throu serval sources, Amulett,Belt and Rings with +X to spells (best would be Sacred,Shadow, Aether but that only works on Amulett and Belt) ofc a Staff or Catalyst with + X to Spells again(best as same as Amulett+Belt). Gear Focus on %Occult, Resource cost Reduction, Toughness, Resistance. At end of Days your dmg should be somewhat around 6-9k which with passive Stuff gets boosted to like 200-300k Crit per Tick which is enough together with other Spells like Solarfall, Anomaly and so on to break down even enemies Lvl 187.

Hard work ahead thou, beeing a Mage takes was more effort than beeing a Melee BE Cookie Cutter.