I play the Xbox console version, so my experience might not be applicable to the PC.
While I've been able to solo all of the uber bosses on Master Level V with a mid-level-geared CM/WW wizard, I've always had a great deal of difficulty with the Kulle/Siegebreaker fight. I recently had to give up after maybe 30 tries when my thumb got tired, so I started looking for an alternative way to farm Hellfire Rings. I leveled up my second witch doctor, fairly well-geared, and today I just knocked down all three uber boss fights in a total of 12 minutes. So, my WD is now officially my baddest toon. Here's what I have.
Paragon level: 43
Bonesaber of Zumakalis with +.27 Attacks per Second, +9% AS, 3% lifesteal, no socket
Homunculus with +Dmg, Vit, Life, low end ZD and Garg cooldown reduction
Zunimassa's Vision, Trail, and Marrow; each with one of AS, CC, CD
Gladiator Gauntlets with CC, CD, LOH
Squirt's Necklace with CC, CD
Witching Hour with AS, CD
Zunimassa's Pox with CD, AS
Unity with +Dmg, CC, LOH
Hammer Jammers (boy are they ugly on the WD!) for the movement speed
Reasonable crafted shoulders and bracers
Four radiant square and one marquise topaz, one RS amethyst; also one marq amethyst in helm
As you can see, the gear is pretty good, but not all top end stuff either. I got all of it from leveling up a wizard and a couple of witch doctors. (Well, a monk, a DH, and a couple of barbarians too, but they don't usually drop Int gear. I did get the Zuni Pox going through inferno with the second barbarian, though.)
All Res 1246
Life Steal 3%
Bonus Globe Radius 15yd
Movement speed 25
Max mana 1021
Mana regen 75/sec
Firebomb - Flash Fire
Firebats - Plague Bats
Summon Zombie Dogs - Final Gift
Spirit Walk - Honored Guest
Sacrifice - Next of Kin
Gargantuan - Bruiser
There aren't too many opportunities to use Flash Fire in the uber fights. (They're great for pulling down trash mobs without wasting mana, and they are hands down the best thing I've found for taking out those annoying tombstones in the graveyard.) You can use it to stand back and ping at the adds in situations where you're waiting for more mana and don't want to attract too much agro.
The goto DPS skill in this build is Plague Bats. (I tried Zombie Bears first, because they do more overall damage, but they cost more mana, miss a lot, and there are less of them, which means that life on hit is less. They also have a shorter range, meaning you have to get closer to the bosses to hit them. I found that I was having trouble keeping up life and went OOM a lot. After dying several times on the Kulle fight, I did some rethinking and came up with the bats.)
Blowing up the dogs is great for some burst damage when you need it (wiping out a bunch of adds in a hurry, for example), and the Garg's stun capability comes in handy sometimes. If you happen to be in range of a stunned boss, you get 3 seconds of unanswered Bat Plague mayhem. The pets also draw agro pretty often, which gives you extra time to reset.
Magda/Skeleton King: Go after the skeleton king first. Use the stairs to help stay out of Magda's arcane beams. Pull down the adds whenever you get a chance, pick up globes. Burn through all your mana on the skeleton king, so long as you have above 30k life or so. Hit Spirit Walk when you run low on mana (or life, the DOT from the bats will restore your life if you have loh and/or steal while you're kiting with Spirit Walk), kite around, rinse and repeat. You can ping the adds with Flash Fire from a distance when convenient. When the king is dead, do the same for Magda. Don't eat too many flies, stay out of the arcane beams. When she puts on the shell, burn down the adds.
Ghom/Rakanoth: I don't have this fight down to a method yet, did some improvising. (The wiz just froze them and that was the end of it.) But, go after Ghom first. (Don't be afraid to plague both of them at once, of course.) Let your garg get his attention if possible, then spam plague bats at him from behind. The gas isn't a problem due to the amount of damage you're doing and your LOH/LS. If Rakanoth whacks you, you should be ok due to the LOH/LS. When you're low on mana, Spirit Walk it back. Use dogs to help blow up adds. Keep them down, switch to them if you need to. If you keep the pillars between Rakanoth and yourself, he won't be able to do his teleport-whack thing effectively.
Zoltan/Siegebreaker: This one gave me the most trouble at first, now it's easier than Ghom/Rakanoth. Go after Zoltan first, in fact, stay away from Siegebreaker. If he goes after you, run to one end or the other. He'll come ahead of Zoltan, your dogs and garg will go after him, and he'll spend all his time messing with them. Run around him, find Zoltan. Stay out of his tornadoes. If you get caught in his slow time bubble, get out of it. You can burn a Spirit Walk to do that at full speed. Once you're in range, plague him until he runs off. You can chase him with Spirit Walk, too. Keep plague bats up on him as long as you can, then run off to the other end. If Siegebreaker comes after you, take off. If Zombie Dogs are dying, blow them up and bring them back. Rinse and repeat until Zoltan is gone. Wait for his stuff to go away if you need to, then start moving in on Siegebreaker from the side. Plague him as long as he isn't paying attention to you. Rinse and repeat again.
Anyway, that's one way to solo the ubers, on the console at least. :)