Lets hypothsize a battle between equal values of Berzerkers and longbowmen (values in shields makes the most sense to me). So we can use 7 longbow against 4 berzerkers. And being the nice guy I am, I'll let the Vikings start first. Ok, 4 Berzerkers kill 4 longbows. Its 6 to 1, the race is not always to the fast or the battle to the strong, but thats the way to bet. That leaves 3 longbows to kill, probably 3 Berzerkers. 4 to 2 isn't a sure thing like 6 to 1, but it isn't bad. So probably a few of the longbows are in the yellow or red after the battle. So , again hypothetically most of the time, we have one berzerker to kill one of the three longbows, again at 6 to 1. Nest round we have two longbows, probably dinged up, to kill one berzerker. End result, we have probably one or two longbows left standing, who need to sepnd some time getting to full health, and no berzerkers.
Obviously when the seven longbows attack first, the 'zerks take serious losses without causing much damage back. Seven longbows likely will eliminate four targets with defense two. I know, the denfensive bonus of at least 10% for open terrain, up to 50% for a mountain needs to be applied, and sometimes cumulatively (across a river, in a city, etc) But this is a thought experiment not a true game test with thousands of iterations and standard deviations with every variable being controlled.
However, what it tells me is that the 'zerker is not too strong, for its cost. It may actually be a little underpowered, and needs to attack from the sea or somewhere where it has the initiative. Or be guarded with better defensive units, if any are available (pikes anyone?). Up against equal values of longbows, or immortals, or probably swordsmen, unless they can attack, the Vikings should avoid combat, if they can. Yeah, four zerks have attack factors totaling 24, but 7 units with attack factor of 4 have a total of 28. Six longbows still have a chance to wipe up the floor with zerkers if the longbows can attack first.