Okay, I'm a bit confused.
First, I don't believe Furious Assault gives you any bonuses above the 1d12. You don't add static bonuses to Sneak Attack or Hunter's Quarry or Promise of Storm (Genasi), and it seems to be in that category.
Second, what is Battle Charge? I suspect you mean Combat Sprint (Barbarian Utility 2), which gives +4 bonus to defenses against OAs during a move. That bonus doesn't carry over to additional actions, so I'm not sure why it's relevant anyway.
Third, I don't understand the attacks you describe. Is the "3d12+5 and a 1d6" from a single attack? All I can think of is you mean "3d12+5 from Avalanche Strike", action point, "1d12+5+1d6 from Howling Strike".
Fourth, at least in the campaign I have the most time in, bosses have well over 100 hp, and probably closing in on 200. (We are at level 7) The single-target damage you describe above would not even bloody a 100 hp boss, and that's only if you HIT, which you will not always do.
Note: I'm not criticizing -- I just want to figure out if I'm missing something. I am about to start a Barbarian myself, and I don't want to go in with misconceptions.
He spent an action point, and also listed the damage bonuses in the wrong order. So, he opens with a howling strike, which is where the d6 and the +5 come from. Assuming he hit with both of his opening attacks, he'd have a reasonable chance of killing a level 2 monster (39.5 average damage), thus allowing him to make his extra attack.