When I get to the point in this quest where I have to kill Mikrul Gaulderson in Folgunthur, he simply won't die. I attack and knock him to his knees (like a quest-related character that doesn't die) but he gets back up with full health and keeps attacking. I've reloaded saves, tried a mod that lets you kill quest-related NPCs and even used some commands to kill him but nothing. Does anyone else get this problem?

You can't use any of the Gauldurson items in combat only neutral weapon items ect...

I think he is the one in the big room that you get to by going down the spiral staircase under a grate that has to be unlocked. I killed him using two different characters. I used either spells, dawnbreaker, and/or an unamed bow.

I remember one of the brothers not taking damage during a battle, yes. It's possible I was fighting with a gauldur weapon back then (thinking something along the lines of "get killed by your own weapons you bastard"), I don't remember. I simply switched to another weapon (probably a silver one, just to be sure) and killed him without too much difficulty.

I just fought Jirik Gauldursson in Sarthaal and none of my blows did any damage to him. I had to use killall in the console to kill him.

You have to wait till the Orb stops effecting him. But in your case it sounds like a bug.

Jyrik is a whole other magnitude of bad than Mikrul. Mikrul isn't supposed to be as difficult as the OP is experiencing - which leads me to believe it's a bug of some sort.

Jyrik on the other hand.... If you get to him the "normal" way (by doing the entire College questline "Under Saarthal"), at one point, Tolfdir will fix the Orb so it's not feeding Jyrik, and then he's susceptible to normal damage (though no pushover regardless). Since I coc into there now (because I have ZERO interest in the college questline after having done it numerous times) it's a tossup whether one of my spells will do enough "interrupt" damage to the Orb to allow me to kill him, or whether I'll have to killall in the console. It's about half and half right now, no clue why some of my toons can fix the Orb and some can't....