The Giga Disc Projectile summons a large spinning sawblade. After a few seconds, it will arc back to the location it was first summoned at. Upon hitting an enemy, it will do its damage and become harmless. It also deals high damage over time if it comes close to an enemy without directly coming into contact, which very quickly adds up if it is moving slower. If it reaches its maximum flight distance it will drop to the floor. If it hits a solid object before its flight time is over, it will bounce in a random direction and then quickly home in back to the initial location. It can also dig through solid terrain (including brickwork) if it comes close enough to it.
This spell has excellent mana drain-to-damage ratio and thus is a very efficient spell to use. If you survive.
Aiming Arc makes this spell "completely" safe to use. The flight path will initially arc towards the mouse, and at the apogee instead of returning it moves very slowly along the its last trajectory until it expires (speed about 100, tested by comparing with Giga Disc + Projectile Energy Field) . It is still active until it stops spinning, so take care not to ruin in to it head first...
Not even the Projectile Repulsion perk is quite enough to stop the self-homing effect of this spell. It'll help, but optimally you'll need two copies of the perk for it to be effective here. Homing is effective in making it more usable, as long as one is absolutely sure an enemy is nearby before shooting it. Having both Projectile Repulsion and Homing, however, allows the disc to both be pushed away from you and towards enemies, making it very safe to use.
You can also use a trigger spell, like Spark Bolt, and couple the Disc with Heavy Shot to limit its range. Be advised that it'll still fly towards you an unlimited distance, even if it's slower.
Path-forcing modifiers such as Horizontal Path or Fly Upwards/Fly Downwards will severely hamper the disc's ability to strike you in all vectors except one. Using both at the same time will cause it to freeze in mid-air after a short distance. You'll need a Trigger to deliver it to the enemy, but this is probably the safest method to use this spell.
Light shot makes it more accurate and the momentum usually carries it away before it can have a chance to home but it also reduces many of its advantages.
Energy shield does not provide protection against this type of projectile
When combined with Remove Bounce Bounce modifier, this spell become relatively safer to cast.