About using speedpotions, indeed, that is a good counter, but often there are people who only pepsi you and nothing more until they consume all of those potions.
But in a simple scenario...
1 Priest, 1 Thief, 1 Archer, 1 Warrior, 1 Mage
VS
1 Priest, 1 Thief, 1 Archer, 1 Warrior, 1 Mage
... All 5 classes have their roles in PVP in a teamwork environment. If a player has ice shield, AC drop is better am I right? These type of situations where each class has their roles. Each class has a play style that should go along with it.
This is why you have seen so many powerful 'small' guilds, able to take on 20-30+ other players. You have classes that can support, constant pressure, support + constant pressure. A lot of players think so small, that they compare class skills in a wrong fashion.
Now that AC drop was implemented. Archers now have an extra role in PVP.
Right time, right place... So many counters to each scenario. That's what makes the game fun! USE TEAMWORK!