first question: why did this battle end in such a decisive victory?

IMO, Bryn made two big mistakes during preparation and set up, so he made it very hard for himself.

big mistake #1: army list. Bryn had few troops that could easily destroy a bunker. He probably should have taken better equipment for the termies (chainfists, kombimeltas) instead of plague marines, some veterans equipped for HtoH, some with meltas and an aspiring champion would have been better. He probably should have taken some fast attack troops instead of the rhinos and the predator and he should have equipped his troops for killing bunkers, not for killing marines (so no flamers and more meltas)

big mistake #2 Set up. Concentrating his force on one flank was good. But he chose the wrong flank, my right flank was the weaker one. So this should have been the point of attack.

In the game itself there weren't many things he did wrong. Moving the plague marines a long the table egde was a bit unlucky, because I had more than 1000 points in reserve - and my reserves would roll over them. They would have been better in the middle of his set up.

Other than that, after his landraider and rhino had been destroyed he had the merciless task of running up into the teeth of a dug in marine force.

But I made some mistakes, too. I left the squad on my right flank where I had set it up. I should have moved them over to the left. So they were fighting out a pointless battle with the squad opposite them. Putting the rhino in front of the squad includimg the future chaplain almost cost me the rhino, but then again had the landraider shot at the bunker, it would probably have cost me a lot more.

Well, Phil is right. I made the big mistake of forgetting the mission target - DESTROY the bunkers. My troops were more put together to take out the troops in the bunkers. That's why I had so many flamers. More meltas and plasma guns were needed (This is partly GW's fault as they are not included on the sprues for choas marines - that's my excuse and I'm sticking to it!). These failings in my chaos army have now been corrected.

Second, why, Oh, why! did I choose the marine left flank for attack. The bunker on their right flank was empty. Well, I'm chaos, I like a good massacre (Blood for the blood god!). But there was also the little matter of the vindicator on that side and I didn't want to lose my Landraider in the first move - which is precisely what happened on the other flank! Moral - don't let yourself be intimidated by big guns!

After losing my landraider and rhino in the first move, my troops, which were set up for HtoH had to lump it across the battlefield while being fired upon by troops in the protection of their bunkers. Any sensible commander would have ordered an air strike! I new I was going to get massacred, but hey! I'm chaos, I had Khorne on my side!

Conclusions :

Bunkers are not so easy to destroy, especially when they're manned and fire back. If you play this scenario, concentrate ALL your heavy fire on the bunkers, forget firing at vehicles - they are not your target!