Attempt to get the sectoids weapon and use it on the disk?
Research done on prior captured equipment indicates that the Sectoid's plasma pistol is designed to conform to their physiology, with a wrist strap too tight for human arms and a psionic trigger that humans cannot utilize. Besides, the one belonging to the sectoid that was killed by the grenade was completely destroyed. In time, these could be adapted for use by our field agents, but for now the technology behind alien munitions is far out of our reach.
C2 Supress sectiod with rifle, C1 suppress disc, c4 shoot sectiod.
Cobalt 4 keeps the Sectoid's head down and Cobalt 1 distracts the Cyberdisk so that Cobalt 4 can take the shot.
Rolled 19 to hit (required <35): Success!
Rolled 9 for damage, reduced by 2 due to toughness.
Final damage: 8
The frail-bodied sectoid gets its head blown wide open by the force of the bullet and dies instantly.
The distraction works all to well, and Cobalt 1 draws the full ire of the Cyberdisk!
Rolled 24 to hit (required <25): Success!
Rolled 13 for damage, reduced by 3 due to toughness, ignored armor.
Final damage: 10
Completely unfazed by the hail of bullets plinking off it, the cyberdisc fires a mighty energy beam at Cobalt 1, reducing him and his equipment to ash.
As for the rest of the team,
Cobalt 2 has 9 health remaining, 25 bullets in his gun, and a grenade.
Cobalt 4 has full health and 26 bullets in his gun.
I'm going to point out now, in case it wasn't obvious, that retreat is always an option.