“Clear the ship of Hive and Cabal combatants until the security lockdown is lifted.”
You will hear from Zavala that you will need to clear out the threats inside the ship before the doors to the bridge will open. With plenty of Hive about, you have some work to do. Expect at least 1 Wizard and a number of Acolytes top come at you at the start of this. From below, you will need to deal with a throng of Thralls that come rushing up. Clear out the hive in this section then head back to the left.
Fire on them and they seem to fall back into the next part of the room, with the ramp. In hard mode, you will encounter a second Imperial Colossus that will start firing on you. You will also encounter Fleet Psions. You will also find the next group of Hive that has invaded that ship.
This group of hive is mostly Acolytes but they are backed up by a Knight and a Wizard. About this time, more Cabal can be coming in from the side opposite the Hive. Finish with the Hive and then clear out this next wave of Cabal.