I managed to get a friend to enter DS9, then I warped to him. My character was VA, and his just made captain and unlocked the mission. I point out that my character was 18 levels higher. Everything else after this paragraph must keep this in mind.
I took a different track and treated this mission more like a STF and used a variation of the tactics I learned from experienced players doing "The Cure" Treating the mission more like a STF makes completeing it much more manageable.
After opening the door, players must grind through the required kills. After meeting that objective, the Assault Legates appear. All players must focus their fire on them as top priority. Then target the Guls and finally the Interrogators and Glinns in that order. Many of the Gils are holograms. As such players MUST target the holoprojectors as they would the Borg Interlink Nodes.
One player should play the runner, preferably one with high HPs, Shields, Armor. That player must avoid shooting at the Cardassians. He/She should pick up the Force Field Key and then run to the consols. Ironically, the Cardassians there will not fight if the player does not aggro them (This has to be a bug as I can be in their personal space and they do nothing!). The other players must draw the fire while the runner makes for the consols.
Once all the walls are down, it is just a grind fest to clear the temple and the exchange in order to recover the Vedek. While it is possible to recover the orb using a runner as done with the walls, the Vedek will not talk or interact with the player until the Temple and the Exchange are cleared.
On Ops, all the players must enter at the same time. Those providing cover must move away from the turbolift in order for the runner to change the codes on the Ops table, Override the controls at Dax's post, then deactivate the Autodestruct on the far wall. These must be done piecemeal as once one is complete, the Cardassians start targeting the running player.
I this mission should only have one difficulty level, at Captian. I reiterate that my character was Vice Admiral, and had two or three Captain level characters providing cover fire. Keeping all instances of DS9 Siege at Captian would make the mission easier to do at higher levels, and allow more players to get in on the action. I would rather have a Captain level character do this mission with 5-10 Admirals rather than only two or three other Captains and have our butts handed to us.
Final words: Treat this mission more like a STF! Use the tactics and stratagy in those mission when attempting this Fleet Action.
I might recommend not having a runner, as I find it simpler and easier to have every weapon shooting the cardassians, since every time I've seen someone run up to down the force field, the cardassians on the other side agro and we get swamped by a horde of holographic gils.
Other than that, the rest of this advice is sound. I find folks would have an easier time at this if they would just stay together and focus on priority targets. Then it's a slow relatively easy march through the station. Though doing that only seems to become efficient with >5 people since then you've got the fire power to mow them down. With <5, people just need to be careful, patient, and bring a healer.
Treating it like an STF is a good start, being methodical about it is certainly better than just trying to AoE everything. Although, it's the AoE guy who's got the best chance of getting top damage, and the only one with a chance of getting anything (slightly) worth the effort of running DS9.