Ghostavo is correct. EMP missiles are the only weapons in the mission which are capable of destroying the Tiamat. The disadvantage here is that EMP weapons can severely disrupt your wingman AI. In this particular mission, affected wingmen might attack the Aquitaine; in later "Capella" missions, they'll actually try to engage the Sathanas fleet around the sun, if their orders are set to "Engage at Will".
The quickest, easiest way to go about this mission is to use a Herc II outfitted with Tempests and EMPs. Whether or not you successfully protect the Oberon is irrelevant. When the Tiamat arrives, make use of the Tempests to destroy its frontal beam turret. This requires some fancy flying, as you'll be engaged by multiple Basilisks and pounded by the Tiamat's flak guns. Fortunately, you'll have an abundance of Tempests, so don't spare the trigger. If your shields drop or your hull integrity falls, take shelter nearer to the Aquitaine to get out of range of the Tiamat's guns.
Tell your wingmen to defend the Aquitaine itself. When the hostile fighters and bombers have been destroyed, order them to attack the Tiamat. Both the Aquitaine and your fighters will be able to inflict heavy damage to the corvette, but more likely than not, the Aquitaine will jump out before completely destroying the Tiamat. What's important is that you manage to disable the corvette before the destroyer leaves; if you don't, it will jump out seconds after the Aquitaine does.
Once the corvette is disabled, you can sit back at a safe distance and diminish its remaining hull integrity with a barrage of EMPs. You'll need to reload your ammunition several times, so for the sake of speed, crank up your time compression to the max when firing and when calling for support ships.
Tiamat objective neutralized.
