Now you'll get to the real reason Akama's here: disabling power generators. Go through the green Orc base to the northeast. Directly north of it is the first power generator. When it's down, Illidan's army can come assault the base. I don't recommend you do this just yet. Head to the west of the green base with Akama to find the second optional quest:
Optional Quest: Siege Bastion

When the green orcs are no more, head north with Akama. There are numerous defenders here, including infernals. You will need to use healing wave a lot to clear them out, but it will save you trouble later. In between engagements, head to the fountain to restore lost health and save your saboteurs' mana. To the east of here is a floor plate that, when pressed, will extend a bridge halfway across the channel. Don't worry about the other side just yet. Head back west with Akama. There is a storage area here with a gate and a couple guards blocking it. Make your way inside to find another 250 gold and a periaph of vitality for Akama. Now go north toward the purple base. A little west of the main entrance is another 500 gold. Head in stealthily and work your way to the next power generator. When it's down, this half of the map is clear for Illidan's army to make their way to the Master of Pain. Take Akama back to the half of a bridge you raised. Soon, the Naga will make it possible for Akama to continue.

For now, your task is to get to the master of pain. There is another Orc base to contend with, as you know, in the northwestern corner of the map. Illidan's army should be fit to handle this if you preformed well in their last big battle. If not, some reinforcements may be called for. Do this just like before. Open with a flamestrike from Prince Kael'Thas and metamorph Illidan. Watch out for the blademaster's specials. If he uses whirlwind, you can tell which is the real hero after he splits. Focus fire on that one after he's done and the Orcs will get a lot less lethal. Once the enemies are gone, have your way with their base. Reinforce your army as you will, but you might not need to. You can banish the Master of Pain while you work on his defenders. When they're all gone, the master will go down quick but will resurrect. Kill him a second time, then hit the switch behind the throne to get some goodies to the west. In the cages are also some fel guards that will join you. When you're done, take your army to join Akama at the bridge. Now it's time to control the Naga.

If your population cap has been worn down a bit here, you can spend some of your gold to reinforce the Naga. A couple of myrmidons would go a long way. If not, don't worry because Vashj's role here is limited. Head north and kill the ships that are in the sewer. You don't have a means to heal your troops so your next stop will be the fountain to the northeast. Use both of your naga royal guard to cast their water elemental spell. Send them in first to draw fire, then destroy the Orcs guarding it, and stop to heal your forces. Nearby are some crates. Destroy them to reveal more gold. To the south of the coins is a ramp that will lead to a lot of tomes that you can feel free to give to Vashj. To the north is a Purple Orc base. Send Vashj and a pair of sea elementals to draw some of the troops out for your royal guard and others to kill. Have your way with their base, then return to the fountain to heal your forces.

Go back to where the orc base was, and you'll find a ramp to the west. Use the sea elementals again to draw fire then set your forces upon the ships. There is a fountain of mana to the west, up another ramp. Send Vashj alone with mana shield to protect her, then send the rest of your army in to deal the damage. Replenish your mana at the fountain and return to the water. Further north there are more ships, and a ramp up to the east. Take it to get even more gold. There is an infernal here. Take him out, then look for the barrels to the east. Shoot them with Vashj to blow down the barriers and grab more gold and a rune of summoning. Use the summoned creature to go ahead of your army and absorb any damage. Your goal now is the switch. There are some tough voidwalkers guarding it, along with some crates. With the summoned creature you shouldn't really take more losses here. Use the summoned creep along with a pair of sea elementals and Vashj to destroy them. Use war stomp with the summoned creep to stun the voidwalkers as often as possible. Vashj's solo run ends here.

Now it's time for Akama to work some more magic. Head across the now restored bridge and go through the last orc base to take out the power plant. Don't attack the orcs that attack you, rather try to hide once the generator is down. Now use Illidan and Vashj's armies in a joint assault against this Orc base. It will be much easier than before with all the extra units. Eliminate the base and go up the ramp last. Use banish on the mistress of pain again and have everyone else take out the defenders. With the second of the guardians down, the door to Magtheridon is made clear. Get the orc warlocks from the cage and return to the gates to assemble your army. If you have any supply left, use your gold to max out at 100/100 or as close as you can get.

Akama will come into play for the last time now. Go in toward Magtheridon and destroy the last generator. Have Akama's forces wait there. Use unholy frenzy on Illidan and metamorph him then walk in with him alone. He'll destroy the defenders easily, and rather swiftly I might add. Use Illidan to draw the next hero in line toward him, then use Illidan and everyone else to kill the last wave. All that will be left is Magtheridon and his defenders, which include a blademaster. Wait a few until Illidan's metamorphosis is cooled down, then use Unholy frenzy on him and Vashj and focus fire on the blademaster, then withdraw back to your army. This will draw everything, including Magtheridon, toward you. Kael needs to banish Magtheridon repeatedly until all his defenders are dead. If the worst should happen and you fail, the only one left to defeat is Magtheridon and you should have a lot of gold left to finish him off.