Things balanced out pretty well after we changed the Power Fist to a Hand of Khorne.
However, the Sorceror seems to have figured out a cheesy force storm tactic…
Well, its PRx10m range, PR hits and 1d10+PRx3 impact damage with no penetration. Since things aren't based on willpower like in DH save its 'accuracy', things aren't quite as bad. Force Storm is one of the few psychic attacks worth using in Black Crusade: Doombolts are just hellguns [eventually plasma] with a chance of headsploding yourself into a TPK. It takes nastier stuff like bolt of change to top it really.
At PR 10, 1d10+30, hitting up to that many times is a lot, but by the time you can do this without extreme partywide risk [or a cataclysmic explosion; there goes all your stuff] the rest of the party is just as bad.
Just as an example, a Legacy Reaper Autocannon with +3 mighty shot, Rage-pattern and Overkill at just Infamy 50 would still have six times the range before penalties, do up to 3d10+16 Pen 9 [average 32.5 pen 9 vs 35.5 pen 0] with each of its six hits, and is completely safe and reliable to use.
Chances are the psyker's force sword is a bigger threat, he just hasn't shown it yet.
Spray/flame psychic powers handle hordes just fine. Or just use a weapon fit for killing hordes on them.
Combine all the answers on this thread for the ultimate answer. Provide Hordes to wear the guy down as well as bad guys like Psykers and dudes with jump packs that exploit the Beserkers weakness. Structure the scenarios so that the Beserker has to "handle the hordes) while the rest of the party races to another goal. The Beserker is critical to parties success because he is holding off the Big Nasties for his team mates and enabling them to compleate mission. say he blocks a doorway and prevents the Ork hordes from penetrating the room and swarming other PCs. Alternately he could keep the impossibly dangerous bad guy busy while the rest of the party solves the Puzzel that will save the day / curse the planet. ..etc
Er we go! Git da Humies! Choot em! Chop em! Waaaagh!!
Being that power fist is his goto weapon send a band of eldar at him(or any group of baddies with a high dodge). At most he can get 2 attacks from his Power Fist with furious assualt. A 5-10 man squad of baddies with 50 Ag, dodge +20, and step aside would really mess up your parties day while not being unbeatable. Add in a force shield on the main baddy to help challenge the berserker and you have provided a challenge. I recommened this as I played a heavy hitting space marine and had that pulled on our group to provide a challenge.
My first idea was also to send in high Ag opponents like Death Cult Assassins, Dark Eldar Wyches and/or Eldar Harlequins. With Dodge +20, Step Aside, Assassin's Strike, Counter Attack and all that funky stuff they should give him a good beating before being hit themselves.