At your dps level, you should be able to kill them quite quickly. For me, the biggest mistake was a too defensive approach; when I finally engaged them without swapping in any defensive gear or massive LoH, it worked right away on MP5. My only recovery was from 2.5% life steal and health globes, at least 1 piece with 5k+ extra heal helps out very much. You already have better defenses than me, I had 550-ish resistances and 32k hp.
The only time you need more is when facing the siegebreaker, for this I used an additional weapon with 3% ls, 950 LoH and a socket for more LoH, and swap to this weapon after Kulle is dead.
I went for overawe mantra, resolve+StI+OwE passives (for the first try I even forgot to swap out fleet footed for resolve and it still worked out). I assume, you don't run your current evasion + backlash built for the ubers? If you do: Don't It works well against huge amount of weak targets, also many of the uber's attacks are not affected by evasion.
When playing solo, the positioning is crucial. Skeleton King encounter is probably the easiest, just stay out of range of Maghda all the time, SK will constantly blink on you. Attack him when he does, dps him down until Maghda approaches, run, rinse and repeat. Use serenity when leoric does his "whirlwind" attack (its animation is broken, but I guess you know that).
Ghom + Rakanoth is a little harder, but not that bad. Focus Ghom first, always fight at the edge of the cloud without taking cloud damage. Always look out for a safe retreat path, save serenity for emergency get-out-of cloud situations, don't use it for dps-ing ghom in the cloud. Don't move around too much, save space by making Ghom overlap his clouds. You can ignore Rakanoth for the most part, he isn't all that dangerous from my experience.
Kulle + Siegebreaker is the hardest to solo, its much easier in a team where one guy can keep the Siegebreaker busy. While you stay melee on kulle, his fireballs won't hit you. Again, positioning is key, however it is way harder to avoid slow bubbles, multiple tornados and a randomly charging siegebreaker. Save serenity for Siegebreaker's grab attack if possible, grab + rock slide or tornado and you are dead. You will probably need multiple tries for this one, as an unlucky slow bubble forcing your serenity followed by grab just kills you.
Try to keep a distance from the siegebreaker, Kulle will sometimes teleport near you, allowing a couple of hits before Siegebreaker comes in. Once Kulle is dead, you should have won. Switch weapon (if you decided to do that) and kill him. It works without switching, if you wait for cooldowns and use health globes, but it takes way longer, is more dangerous and quite annoying.
At your DPS level you will probably need about 3 to 4 minutes for the other 2 encounters each (I kill them in about 2 minutes each@ mp5 in a solo game, 100k unbuffed), so you can easily afford taking a little longer on the last one. If your NV timer reaches 10 minutes, you should consider refreshing it.
What skills/build are you using on the siege+kulle fight. I've had most success with exploding palm instead of cyclone, Will try a bell build, need more burst dps I don't have time to wait for cyclones and I'm certainly not able to keep it up during the fight either.
you need both weapon with life steal and maybe with life on hit too on weapon or ring/amulet.
I thought this myself, but I found the solo fights to be easier, if I don't put on extra life steal or LoH. In team games, thats a different story, it helps your teammates a ton, if you can just facetank the ubers and don't have to move around much, assuming they are mostly glass cannons in need of a tank (true for most of my friends I do ubers with).
As mentioned, I didn't need more than 2.5% life steal for anything but finishing the siegebreaker. Just solo-ed the ubers again on mp6, without too many problems.
Also, for the Kulle/Siegebreaker encounter, movespeed is very helpful. 12% is absolutely minimum, 24% helps a lot, and I had the feeling it was easier with fleet footed for 34% movespeed instead of resolve for 20% reduced damage. You are just constantly running around, avoiding tórnados, rock slides, the ever-annoying slow bubbles and the quite speedy siegebreaker. This run is about 75% running while Kulle is still alive, for me. Avoid stuff until you find an opportunity to hack away at kulle safely for a couple of seconds, blind as soon as Siegebreaker approaches, keep whacking for 2 seconds, run again. Resolve is more helpful for the other encounters, but since those are easier anyway, I will probably go fleet footed again next time.
You cannot out-heal their combined damage output + sick disables + reflect anyway, so don't even bother (in team games, when you can split them up, this is way easier). Maximize your dps, so if you see an opportunity, you can do a ton of damage. With 100k unbuffed dps on MP6, it took me about 10 minutes (and 1 failed attempt), so you should expect about the same at 80k dps on MP5, also as mentioned, you have way higher survivability than me anyway.