max S and WS with maximal rolls or full point-allotment and Perfection[strength]+martial-prowess could be 80 each. Crushing Blow needs a 70+ to grant +4 damage, so we'll assume this too. Now throw in the +20 from power-armour, or +30 from a Terminator unit. If we're talking terminator, dodge is gone anyhow, so toss in high-grade muscle augmentations for a total unnatural strength of +8 [more still if mutations].
Already we're looking, for the full termie setup with implants, at turning that base best-quality powerfist from 2d10+1 Pen 8 to 2d10+1+22+12: 34 bonus damage.
Even higher pen and damage if that power fist becomes a legacy weapon [its certainly getting its owner's fame right now].
Now then. As with most marines, you should be pulling out the heavy weaponry. I doubt he's that much tougher than the rest of the party, and most things, despite his likely insane charging speeds, are going to outrange his melee capabilities significantly enough for at least a turn or two. And, I'm sorry but its OBVIOUS, painfully obvious to anyone, that HE needs to be taken down NOW before he gets to them. The rest of the party might use cover, blocking LoS, and so on and so forth, making heavy weapons less of the threat they really are.
Anyone bothering with anything with less output than Hellgun+mightyshot against even a starting legionaire deserves his fate almost as much as someone rushing into combat with what is so obviously an armoured dreadnought-violating monster who probably looks halfway to a bloodthirster already. People should be RUNNING if they can, or An-Heroically calling fire down on their position if he does get to them in a vain attempt to save the rest of their own squad.
When he charges, people shoot from cover. When he gets close, people run away. Psykers want nothing more than to turn this guy 'round on his allies, and Teks would gladly pin him where he stands with gravy-guns while drawing straws as to who gets to try to Servitor the guy. Alternatively Haywire weaponry can calm him the **** down, shutting off his power-fist [dropping 11 damage the 8 pen right there] and sealing him in his own armour at least momentarily.
But perhaps most importantly: If your party currently isn't dealing with world-changing matters when they've obviously got this much XP behind them, its… actually possible that your berserker's player is not actually the one at fault here [as I've shown his damage could be substantially higher], but rather the others who could have been half-assed about where they go or what they use their characters to do.
For example, a demagogue that ISN'T corrupting entire armies at this point because he'd rather fight in melee would be doing it so wrong that even Grandpa Nurgle gets *pissed off*.