Dude I just tried this about 60 times and we just couldn't get it right. I killed Skolas once but nobody captured mines. Other than that we never killed him using 3 maxed gjallarhorns. I have no idea why but he would have over a 1/4 health by time the mines spawned and all of the enemies. Pisses me off to be honest.
We just beat him. (My second time) and when he went down we had already been poisoned for like 10 seconds and one of us had already started running to a mine that had spawned (well after the critical objective had been announced). It can be done. It just take perseverance and level heads :) Frustration makes it a lot harder! Keep trying!
It's highly frustrating. It's even more frustrating when random and stupid deaths keep happening. Like players not running to mines, blowing themselves up, trying to melee a captain, not knowing to say how long they have left to pass off the poison, and about 40 other things! Man I saw it all for about 6 hours! I'm not sure if it was my team or if most of this stuff is normal when you haven't done it. All I know is 99% of it went wrong. Lol