I just solo'd this on my warlock and here is what I saw...
The admiral himself is a straight tank and spank... doesn't really do anything special. He hits my void (16k armor) for about 1500ish a swing and does a wirlwind every so often and hits for about the same damage. (didn't hit my void as hard because of avoidance but I'm imagining it would be about the same).
At about 50% hp, he will put up his shield and you will need to use the Lord-Commander's Nullifier to get rid of that shield. He becomes immune to everything while the shield is up, so you need to use the nullifier quick to continue dpsing him.
I would say within a minute of taking off that shield (it might have been a certain % of life that triggers this but I wasn't watching close enough)... he transforms into Dreadload Mal'Ganis and the fight changes slightly.
Once he transforms he has a few other abilities. He hit roughly for about 1600-2000 per swing and has typical dreadlord ability. He casts Carrion Swarm which did about 300 damage / 2 seconds, Mind Blast every 10 seconds or so for approx 2k shadow damage (he turned and hit me with just about every mind blast then resumed attacking the voidwalker) and he does a Sleep on about a 10 second cooldown as well and only lasts for about 3 seconds (would sleep me and continue attacking my void).
Something to note as well, when he transforms, his maximum health jumps to 384k but you continue fighting him from the hp he was at (roughly 40% or so when he transforms). I didn't notice how much hp he had remaining when he transformed but I did notice his max health spiked so my guess would be that his remaining life scaled with that as well.
He can be feared in his first phase but I found he evade bugs often and resets if he gets feared down the ramp.
Took about 15 minutes or so to kill him. Remember that all the physical damage about was done to a voidwalker with 16.2k armor... so if your a warrior... scale the damage by whatever armor you have.
Hope this helps!