Ok finally got him and Ick & Krick down..
For forgemaster i did the following, (compiled tips from here)
1. Have my team at the botom of the steps and to the right(or left does not matter as long as you start to one side)
2. Tank at top of stairs, aggro one of the small mobs to get forgemaster aggro'd, you could Turn him so he is not facing your team, preferably with your back to the stairs. Though i did not, my problem was with boulders not frost damage.
3. Ignore the mobs until the first time he runs to the forge, when he runs to forge kill the mobs.
4. "Saronite Boulders" if they go at you team just strafe them to the middle, if its the tank just switch sides of the stairs, or strafe a little left of right. Each time he targets your team move them over using strafe, when your over the other side of the stairs, move them forward 10 feet to miss the boulder then strafe them the other way as need be.
5. "Frost Debuff" When he goes back to forge this auto clears, nice and easy.
just by doing the above i managed to clear him on first try with the tactic, after wiping around 5 times trying other things. The key was the strafe movement at the bottom of the stairs, which made this a much easier fight.
Here is a digram of what i'm describing;

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