I got my first exotic sword around the same light level as you are right now, and we needed to get two of the Knights. I think we wiped once at the end, so I think you can pull it off. Worst-case scenario, do two runs, one for solar, and one for arc and void. The solar knight has a ranged attack, so if you kill him, that will make arc and void much easier, then you can do it again and kill the arc and void knights, and only have the solar guy to watch out for (in addition to the Dark Blade, of course).

You can do the strike all at the same time. All you need to do is - as Raver Virus said - keep the 3 warden alive while you lower Alak Hul's health. We actually tired doing it but we ended up killing one of the warden. I think it is doable but I think it's easier to just to it one at a time. Btw, if you'll be doing 3 wardens at the same time and accidentally kill one of the wardens, you can just wipe so you can do it again, you'll respawn at the last checkpoint so don't worry. When it happened to us we didn't think of it right away so we just did the whole strike again. Hope this helps!

You can do all 3 at the same time, but it will be pretty difficult staying alive. Remember to kill the wardens 1st, then you will have 30 seconds to kill the dark blade.