Large zergs are not difficult to deal with as long as you are structured in your approach. You do need enough knowledgeable players using the tools necessary to dismantle large teams however. Over time it becomes an attrition thing because structured teams are difficult to maintain over time. For example, if you rely on a Mesmer and Necro with excellent well/null field placement and they log out to eat, your capacity to remove boons is very much diminished.
A server that actively recruits can keep throwing numbers at you until your structure collapses. You nevertheless pay a big price for zerging up and you can capitalize on such weaknesses under certain circumstances. If you concentrate enough players in one place on the map to overcome all individual weakness in combat, you naturally cede much of your ability to react to concurrent threats on the map. Your scouting and ability to guard supply camps/walk dolyaks and man defensive siege is diminished because so many players that should do these things are running in a giant pack and are required to do so for team synergy.
There are also very significant weaknesses to running large groups that move in a tight formation stacking boon duration. They are extremely vulnerable to targeted use of static fields, frozen grounds, wells (of corruption/power), null fields, sanctuary bubbles, rings/lines of warding etc. They are very vulnerable to mass control (knockdown, stun, daze, chill, cripple) which forces a team to separate/break apart so they cannot share buffs or combo off each other effectively.
I think that boon removal is a little too limited right now. Targeted boon removal needs to be a bit more prevalent and there needs to be a method available to some or all classes to remove or “pierce” boons like stability which totally negate control effects. Running in larger groups just lets you have greater stability uptime compounding the problem.
Nevertheless, it is easier to structure small teams than it is to structure very large teams. Another great weakness of zerging is that controlling every player and using every skill to best effect is often impossible. Getting large numbers of players to move correctly, spot and use terrain advantage is harder than if you have less players desiring structure.
Ultimately I think it is best to take an iterative and fluid approach to team composition. You cannot always have the tools you need for every conceivable circumstance so you need to make do with what you have. I think education is of primary importance since the goal is to inspire as many players as possible to play well, to not treat defeat as failure but rather as discovery of methods that do not work.
On one level zergballing works because having so many players in a tight formation increases the likelihood of benefiting other members in the team through synergy/sharing even if it is accidental. But overreliance on same breeds in weakness and zergball teams like this can be dismantled or at times avoided altogether in order to accomplish objectives that matter. Have you ever seen 30 enemies charge into a null field/static field/frozen ground? I have. You get alot of loot bags if you are capable of controlling the movement of your enemy (by seeking terrain advantage, using cheap AoE, marks etc to herd players into avoiding them).
I would encourage dueling/sparring. I think it is important to play every class so you are aware of the limits of what can you can achieve and what contribution you are capable of making in team.
I’m not keen on high levels of structure like in Guild Wars 1 because it is a double edged sword. GW1 was insanely structured to the point where if you didn’t have one very specific person playing one very specific character build, your team build does not work. GW2 calls for a more fluid and adaptive approach but it is sadly not talked about enough.
The talk is mostly about winning games via attrition – usually by throwing more resources (supply and manpower) at your opponent than they can at you, thus wearing them down faster.
