My approach for FW was to wait until I have better heroes and equipment for the faction/stages. I did farm what I could, but concentrated more on the dungeons to get better equipment and on arena to bring up the Hall.

This might not help you but perhaps it gives you another perspective. 

At this early stage (1 month) I also tried to bring up as many champs as possible to lvl 60 first...with time and equipment, the first bos should not be a problem anymore.

The thing is I actually have 1x lvl 60 and 2x lvl 50. Not being able to pass the first boss with that sucks, but that's the way they boss is made. No crowd control, no passing it.

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Gravechill doesn't have masteries, but UDK and FB do.

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Running the boss on auto, they actually one-shot my UDK every time. 

So I tried to manual the boss and always killed the minions first before hitting the boss. It is essential to kill at least one of them before the boss can start his ally attack. 

I tried to save up all major abilities in stagfe 2 so I have them on stage 3.

Speeds: UDK: 148 Frozen: 179

Perhaps that helps.

I think I'm just not strong enough yet. My idea was probably pretty good, but I don't have the means to see it through.

Brute-forcing it by focusing the minions is not an option for me yet.

Should probably clarify that it comes down to gear. I've won a few tournaments which gave me sets of rank 6 gear, but that doesn't mean it's any good.

Take the advise Matrim gave in the first answer. Concentrate on something else first. You can take dow this boss when you have more champs that are worth lvl 60, and when they are lvl 60.

Yep :) that was the idea.

While I have you here, instead of starting a new topic.

Does the blessing Crushing Rend and ignore defense from gear stack? That could be an interesting way to amplify damage on a nuker.

As I don't do datamining from the games code, I don't know for sure, but from the ways the effects normally work, I don't see any reason that would prevent it from stacking, so I assume "yes".