After the Elite has gone down, you should have the room to yourself momentarily. Unfortunately, you'll soon trigger the appearance of two more, so you'll again want to heal up your troops before moving through the substation room. When you're ready to move on, put Sev and one of your other soldiers in the two sniping spots in the room, but leave the anti-armor position vacant for the moment. The substation terminal is located on the small central walkway suspended above the middle of the room, so head up there and start slicing the console yourself, but as soon as you hear the sound of bugs, cancel the slice and turn around to take them down.
With two snipers, you should be able to concentrate your fire on one of the Elites and take it down relatively quickly, then repeat the process on the other one. Someone's going to get roasted, though, so don't be too worried if a squadmate goes down; just finish off the second Elite before healing everyone up and getting ready for the next phase of the fight. When everyone's set, place the third squaddie in the anti-armor position and slice the terminal yourself. Doing so will open the door on the floor, but don't head through just yet.
Before you enter the corridor, take stock of any ammo supplies in the room. There should be a set of Thermal Detonators near the door through which you entered (assuming you didn't pick them up before), as well as a box of EC Detonators near the door that you just sliced. Heading into the corridor will trigger the appearance of another spider droid, so it'll be useful to know precisely what you have to use on it before you try to take it down. Obviously enough, saving your game at this point would be a good idea, because a single spider droid can kill you in just a couple of hits.
Luckily, though, your squaddies will be able to distract it from you with their fire, and indeed at least one or two of them will probably keel over dead before the droid gets destroyed. It'll probably start firing at the sniper near the bacta dispensers first, as it'll see him before it sees the anti-armor position, so take advantage of that by unloading any spare thermal or EC grenades on it as it walks up the corridor (assuming you still have boxes of them left to pick up; the objective is to have a full loadout of grenades after you kill the spider droid, not to pick up everything and use them on it for only minor damage).
Beyond that, your anti-armor position should be able to do most of the damage needed to take out the spider droid. Feel free to contribute what you can by sniping the ocular cluster from a distance away, or by using your anti-armor shells. It's probably better to save the latter for the upcoming fights, though.
Access Main Elevator
When the spider droid has been dispatched, pick up any ammo left in the room and completely heal your squad. The only remaining room before the Main Elevator is fairly packed with droids, so you may want to slice it and keep the squad back before you chuck in an EC grenade to temporarily disable the first SBD within; this will let you send your squadmates into the room in force to blow it for good. After the second SBD is off to that great robotic afterlife, return to a bacta dispenser to heal them all before riding the elevator up to the bridge.