Question about Sauron's army during the War of the Elves
 

Was it just sheer numbers and the power of the Ring, and him personally took command of the army, that he could destroy the last stronghold of the Noldor? Could thousands of orcs, easterlings and haradrims defeat a whole kingdom of high-elves?

Assuming you mean the Second Age conquest of Eriador, I would say numbers alone tilted things Sauron's way. His being in personal command would have driven his forces to a hatred for their foes and a disregard for their own peril as well.

Sauron had also been to Ost-in-Edhil. He knew the ways in and out. Given how much the Gwaith-i-Mírdain liked "Annatar", it's entirely possible he helped design their defences.

I actually don't think he would have had to work too hard, either. The name "Ost-in-Edhil", 'Fortress of the Elves', plus the general Noldorin practice seen in Beleriand, suggests Celebrimbor probably abandoned the countryside and forted up in the city. That's great, it's a wonderful city - except that it sat between two rivers, and Sauron had an army perfectly capable of damming said rivers. (Sure, he'd probably lose a few Orcs, that's dangerous work - but so what?)

Depending on how he did it, mucking with the rivers would either remove a vital component of the city's defence, starve (thirst?) it out - or, if the dam is downstream, flood it. He could even build an upstream dam and then break it, to throw a cataclysmic flash flood at the city walls.

None of which would be possible for someone who cared about their soldiers, which is why Celebrimbor didn't plan for it. Or, well, maybe he did - but that plan was the Ring of Water, and he'd sent that off with Galadriel. Whoops!

(Honestly, though, it still seems to have done better than the last island-fortress Sauron conquered; he took Tol Sirion without the defenders even standing and fighting!)

hS

A bit off topic, but who were Sauron's generals during that war? Was the Nazgul already in his service and what kinda state they were in if yes? And could it be possible that a easterling or haradrim warlords lead Sauron's orc army as well?

Definitely not the Nazgul - Sauron's invasion of Eregion is when he took the Nine Rings that he used to create them! It would be another five hundred years before they first put in an appearance.

Tolkien Gateway gives this description of Sauron's preparations for the war:

We know that Orcs served as captains (I think this is the highest rank Tolkien ever uses to describe war-leaders, besides some oddities like Marshal of the Mark) under Morgoth, and we know that Sauron's forces in the War of the Ring operated as discrete units (ie, the Haradrim didn't fight alongside the Orcs, but showed up separately). Given that Sauron was present and in command, I think we can assume segregated armies, each with their own leaders who Sauron ordered around as he saw fit.

Not sure whether that would include an army of trolls - I've always pictured them more as solo warriors, or maybe living siege weapons. It's also worth noting that "mostly" on the Mannish armies - Sauron may also have drawn on the ancestors of the Dunlendings, who were at least partially under his shadow.

hS

I can see trolls as more siege weapns and elite force if you will, since they are so big and strong. But since he didn't have the olog-hai yet, he must have use dark clouds to block out the sun, wherever the trolls went?