While kotezot's answer is the best course of action, there is an ugly alternative.
Sacrifice a crew member. Teleport them over to start destroying the shield generator. Then do everything you can with your other weapons to bring it and keep it offline. Crystals, Mantes and Rockmen are best suited for this task with their improved attacks or protection. Humans and Slugs can do some damage, but won't live to return. A single Zoltan or Engi won't do enough damage before suffocating.
Unfortunately, you'd have to expend two crew members to pull this off, leaving you in a dire situation.
This reminds me of an episode of Start Trek: TNG, Thine Own Self.
On the Enterprise, Troi asks Commander Riker if she is allowed to take the Bridge Officer's Test. Riker wonders why, and is told that she has considered it off and on for over two years, but was motivated by her class reunion, as well as being in charge during the events of Disaster. She is allowed to take the test and does so at the holodeck.
Geordi La Forge and Worf are the senior officers in the simulation, where the insulation of the antimatter chambers is breaking down. Troi has to make fast decisions, but fails. Riker tells her that all parts of the test went well for her, except for that last and most important part of it, resulting in Troi failing the whole test.
After having entered the holodeck, Troi starts the test's simulation again. She is in engineering, and the antimatter is about to leak out of its container. She asks La Forge if he can repair the damaged parts. The Worf hologram says that La Forge never will survive the operation, but Troi, well aware of this, orders the holographic La Forge to repair the damage anyway. As an anguished Troi watches him enter the turbolift, Riker, having known she would not give up so easily, enters and congratulates her on passing the test. She immediately deduces that the point of the test was to see if she could order someone to their death, which Riker confirms. Having known this was part of being in command, but having still hesitated when the time came, Troi admits that he may have been right about her being ill-suited for the job - to which Riker counters that she passed with flying colors; she had tried every possible solution she could think of, and when nothing else worked, she made the painful but necessary decision required to save the ship.