Ok check it out. This is the order for skills low level untwinked.
-Put only one point in skeleton mastery (trust me just max this later).
-One point in amplify damage
-One point in iron maiden
-One point in clay golem
-One pont in golem mastery
-One point in corpse explosion
-One point in raise skeleton mage
-One in prerequisites
-Then ALL THE REST IN RAISE SKELETON
When you get to duriel you should be around level 20ish, so by then your skeles will be very strong even with only 1 in mastery. Immediatle cast iron maiden on Duriel. He will usually prefers to attack your golem before he attacks anything else, so bring a ton of mana pots and keep spamming that clay golem and iron maiden on him. he may destroy all your skeletons eventually, but you should be able to bring him down with just the level 1 clay golem and iron maiden.
Now Mephisto/Diablo/Baal
You should have a point in decrepify and life tap at this point. Also a point in summon resist. Make sure you have a cold using skeleton mage. Cold+clay+decrepify absolutely paralyzes these guys if you can get it all casted before they lay waste to your troops. If you think you have them slow enough without decrepify, then try casting level 1 life tap. it helps your skeleton warriors very much.
I reccomend maxing raise skeleton as soon as you can, then getting corpse explosion going, and enough amplify damage to keep up with your needs. After you do those to a respectable level (like 20 for ce

, then max out skeleton mastery. The reason I say skeleton mastery last is the difference between level 1 mastery and level 20 mastery only doubles your skeleton damage. You get much heftier gains by adding to raise skeleton. See the pet calculator and it's all self explanitory. At this point you need to decide if you will maximize skeletal mages, or maybe maximize golem mastery, or alternatively dim vision. Mages= more killing power, golem mastery= nice iron golems with very high life, dim vision= good defences.
Also, against bosses, your mercinary is absolutely your best minion. Try to get him equipment with crushing blow. Strength runeword is easy to make (amn-tir), but there are so many more. Crushing blows will take huge chunks of life from bosses.