Bear in mind that the selected difficulty affects your decision making.
The cyberisc has two modes, obviously:
Saucer: increased movement. I've been told on Classic they get crit immunity here.
Combat: it puts out lots of damage, but is very vulnerable.
Because of the high amount of HP a disc has, you need to focus fire. But you also need to be wary of it's helper drones, which can heal it. These are extremely easy to destroy, so don't waste high damage attacks on them but do take them out first unless you are extremely confident you can "alpha strike" the cyber disc. There is nothing more painful than reducing one to low hp only to have it withdraw and get healed a bunch. Behavior I have witnessed on normal.
I don't know if it is particularly vulnerable to a type of damage (I don't think my autopsy said, I'll double check when I get home). Also, do not bunch up. It can hurl plasma grenades and tends to do so when you give it a nice cluster of 3+ targets.