Let's picture a medieval world, where a tribe of Viking-like mercenaries live for the thick of the fight, the blood nourishing the ground, and the honour of the gods of war. So they are equipped, trained for skirmishes and wars. Their equipment would be Viking like, for instance:
- Round shields along with short swords and spears, especially used for battle formations.
- Axes, but probably more used as all-purpose tools than on the battlefield.
- Longbows and possibly javelins for ranged options.
- Thick leather and fur armor; less resistant than full-plate ones but still enough to prevent a cut or two and lighter to wear. Chainmails are also there to supplement the defense of the frontline.
- Iron helmets (horns not included)
- Some horses
Now, a small part of this tribe goes much further in this battle mindset to an almost fanatical aspect of themselves. Being stronger, faster and more endurant than other soldiers and having access to the same equipment, they take enhancing drugs to be truly one with the battlefield and have a seat among their gods when they will expire. With that comes an increase in adrenaline, loss of fear, acutened senses, and much stronger pain resistance.
Note that drinking the decoction is quick and the effects start to take in fast too, like 1 to 2 minutes at most, making one ready against even surprise strikes. Depending on the quantity taken, the drug can last from 1 to at most 4 hours. taking for longer than that is dangerous to the health and have nasty adverse effects.
The big downside is that they lose control of themselves : Apart from other berserkir, They barely recognize friends from foes and, due to their excitement, they have a difficult time refraining their pulses to charge the enemy, unable to follow a sound formation. Your perfect berserker, in short.
Now that the setting's set, what tactical advantages can be taken from such units, in a battlefield?
For the purpose of this question, we will assume that the enemies have approximately access to the same kind of equipment and troops (minus berserkir). Also, until they become frenzied, berserkir can and will follow orders. However, they refuse fight orders if they are not frenzied : If you send them on a charge, they will take their drugs beforehand.
Having thought a little ahead, sending them with other units looks like a real bad idea for obvious reasons, but sending them alone on the frontline isn't any better. Indeed, battle formations are exceptionally strong against a disordered charge, as the attackers literally impale themselves against the enemy line. I can picture quick skirmishes and guerillas on the logistic lines would work quite well, but during a direct confrontation, hmm... I have a hard time seeing how and where one should send them to get the most out of them.
So what advantages can you get from them on the battlefield? This comes with many subsequent, related questions that may be interesting to look at to find an answer. For instance, when should they be deployed and where relatively to other troops? What would be their favored environments to fight in (forests, mountains...)? Would they have good match-ups against specific troops types?