Ok, the secret to fight this ship, and nearly every other one with big one shot guns is:
Stay out of the front arc!
Romulan battleships in particular are most vulnerable and harmless from the flank side.
Stay away more than 5km.If you are snared anyway you can use Attack Pattern Omega, Polarize Hull to resist the beam.
You can force your way out of it with evasive, power to engines and other engine boosting abilities.
And you can stun and disable the other ship and outright break the reactor beam with Photonic Shockwaves, subnucleonuc beam, scramble sensors, jam sensors, a tricobalt hit, ...
And my favorite point defense system: Tractor Beam repulsors. That will push her out of her own tractor range and has the nice effect to destroy any iincoming plasma heavys because it has 100% accuracy.
A single tractor Beam of yourself is useful to prevent yourself getting into this Situation in the first place.
Shields against plasma attacks won't help you here. They only work against energy based weapons. Those torpedos deal kinetic damage.
So the best shield against it should be a resilient shield with a high cap
because it reduces bleedthrough damage and while you are at it, equip an armor console against kinetic damage.
Should be a piece of cake now to beat the mission.
Aye, a PBAoE is very useful in this fight - the "stay beyond 5km" rule is sound, but she likes to pop Evasive Maneuvers to rapidly advance on you and, if you're a cannon build, you'll be losing a lot of your own DPS output by staying away - she heals pretty quick so you need to keep the pressure up. Tractor Repulsors will do her a bit of damage as well as keeping her and her trios of torps off you, Photonic Shockwave works OK as well, but Tractors stay on for a bit so if she launches a single torpedo straight after, you might snag that one as well.
Just try and keep out of the forward arc as much as possible, not just because of the guns but because of Viral Matrix... you really need your powers for this fight, and having engines or all Auxiliary-linked powers go down at the wrong moment sucks.