I've just played my first game in this beta against an Oak who got spirits (a hard AI), and I have to agree. Those things are insane. I was playing Erebus, and I dodged the bug that stops you from getting Night Walkers. Didn't matter; I never saw them do any noticable amount of damage or survive for long at all, even at max level. Meanwhile, if I didn't start fleeing (ideally teleporting) away as soon as I saw Oak, I was guaranteed to die in under five seconds once his spirits swarmed me.
I tried countering it. The only counter I could think of (perhaps I'm just too dumb to play this game?) was to load up on armor items. About 15,000 gold later, I had the best non-artifact armor I could buy, but that only got my damage mitigation up to 65%. The spirits still chewed through me. In one attack, Oak waltzed into my base with his army, wandered around aimlessly for a while, then noticed me, slaughtered me in a few seconds, and proceeed to level six towers before I respawned. Then he got bored and went off to capture some flags (if he'd been a little less ADD he would have destroyed my citadel about ten-twenty minutes in; that was one of the only advantages I had). I also tried loading up on damage items, on the grounds that if I could kill Oak, his spirits wouldn't be a problem. I got him down to a few hundred HP once or twice, but I always got killed first. Nothing Erebus has -- not his minions, not Bite, not Mist -- could dent Oak fast enough or take down those flying terrors. I tried hiding in Mist a few times, but that just delayed the inevitable.
I did win in the end, but only in the lamest of ways. Oak's teammate suffered from the AI Braindeath, so I got some gold by killing him repeatedly. I used it to buy priests and angels, portaled into the AI base, and ninja-grabbed their flag with a Warpstone. Oak's spirits could splat me in seconds, but when they weren't around I had enough armor and HP regen that I was able to knock out the entire enemy base from within, solo, without stopping to heal. Once reinforcements started coming through the portal, their citadel fell quite quickly, while Oak was still wandering around re-capturing flags that he didn't need.
I hope that it isn't overdone (IMO the other generals' minions are too weak), but I think something really needs to be done about Oak.